I am having the same problem,

I bought the "Stealth plug" it said it was the easiest way to hook up your guitar to a computer, yeah right, I mean it was easy, the only problem is I had to change my ASIO driver to the stealth plug which brought the whole thing crashing down. Changing your ASIO driver is NOT the solution because sure you can change ASIO driver to whatever you are using and hear your guitar, but as a result of changing your ASIO driver from your sound card to whatever ASIO driver is for your guitar all you will be able to hear is your guitar, you wont be able to hear your track because you changed your ASIO driver from your sound card to the ASIO driver for your guitar. Trust me I have tried almost every way to do this, I mean if you cant hear your track while playing / recording your guitar whats the point, nothing you play will be in time with your track because you cant hear your track because you changed your ASIO driver and disabled your main soundcard so doing this is not a solution.

Other things I have tried so you dont waste your time, I realize the guitar needs to be pre-amped in order for it to make any sound at all, I have tried hooking up to a guitar amp then out of the amp into a studio soundboard then into the input of the sound card, from that I got massive feedback, then I tried skipping the sound board and pluged from the amp straight into the saoundcard, again it worked but with massive feedback. I use a M-audio fire wire 410 as my soundcard, it has 2 microphone inputs that are preamped and it has two plugins for it, a mic input but it also has a quarter inch input, if you have a quarter inch input on your mic input on the soundcard plug your guitar straight into that slot, now a guitar only emits a mono sound, so in Cubase just open an audio track, choose mono, and then click the little button on the audio track that has a speaker on it and boom you should be able to hear your guitar, but only a clean guitar sound, no distortion of FX or anything else just clean guitar, but I get no feedback, well maybe a little feedback, kinda like an air sound but I could deal with that because once the guitar is integrated into a track with all the other sounds going you would not be able to even hear the air feedback. On my soundcard I have a volume nob so I can turn the guitar volume up or down, depending on how hard you are hitting the string on your guitar to avoid clipping.

So yes I got my guitar to play on my computer but as I stated only a clean guitar sound, which if you play guitar you know this is not a solution because you can only play clean, no distortion or Fx etc.... also if you try this make sure you are pluging your guitar into the mono track on your sound card, if you have not already chosen a mono track on your sound card then when you open up a mono audio track in Cubase the sound card will usually choose its own mono channel, so plug in to the mic input, if it does not work then try the other mic input, there should be 2 mic inputs or more depending on your sound card. I also have pedals for my guitar, distortion and others, I tried hooking those up, I went from the guitar to the pedals to the mic input on the sound card again got massive feedback so that did not work either.

So basically I have not yet solved this problem, I have a couple ideas that I have not tried yet because I need to save up and buy the software, I guess I still own the stealth plug and its software so I can try 1 of my idea's, I was thinking since you can plug a guitar into the mic pre-amp input and get a clean guitar sound maybe I could use the stealth plug software on the fx channel on the audio track and be able to use all the stealth plugs pedals, fx etc.... I was thinking of getting the guitar rig 3 because it has waaaayy more stuff in it than my stealthplug software, but I'll try using it on the fx channel on the audio track and let you know if that works.

My 2 other idea's I have is to see if you can use the stealth plug or guitar rig 3 as a slave to Cubase the way some people slave Reason and other DAW's to Cubase, my 3rd idea is to get a microphone and just mic the guitar amp and just play through the amp, my 4th idea is honestly to contact Skazi and see if he will give out the information on how he does it because he seems to do it just fine.

Because your rack mounted guitar processor has a USB plug on it shows it is able and was made to be able to hook up to a computer, but you dont want to change your ASIO driver because you wont be able to hear your track and thus all your guitar parts will not be in time with your track, you want to be able to play along with your track so you have full control and lay down the riffs the way you want them. Check out the website of the company that made your rack mounted guitar processor if there solution is to download a new ASIO driver skip it, your next option is to see if they have stand alone software, if they do try that and you should be able to open the software separate from cubase and then see if you can slave that program to cubase. Shitty thing is this should be so easy, you would think someone would have come up with something simple and cheap to solve this problem by now.

Anyhow, I am getting ready to work on one of my tracks right now, I'll try using my stealth plug, which is like a much smaller version of guitar rig, as an fx channel on the mono audio track and let you know if it works.

Also know that once you, (we), get it to work when you record the guitar in mono you can always take that mono track and do a audio mixdown to a stereo audio track.

On a side note I know that guitar companies are working on a new type of guitar that has USB and fire wire inputs, and an internal soundcard and pre-amp that will digitize the guitar before it reached the soundcard, making it so you dont have to change your ASIO driver I have heard the guitars will be pre-powered by either batteries or actually having a plug that plugs into the wall, this could pretty much turn the guitar into a synth it would be pre-powered and digitized before it reached the computor, just like a Virus or a Nord or like most any synth. But you know these guitars are going to be super expensive when it comes out.

Trust me man I want to solve this problem as much as you do, I have been playing guitar for about 19 years and would love nothing more than use it in the psytrance I write now, most people dont understand that for a guitar player that also writes music on a computer, psytrance or any electronic music, there are processors and all sorts of stuff that we can use to change our guitars into almost any instrument, so for instance, I can not play piano or keyboard, but I can use processors to make my guitar sound like a keyboard, violins, congo drums, almost anything. So all these different instruments I do not know how to play I can now play on my guitar and then essentially I know how to play all those different instruments, plus you can do fx and all types of other stuff.