Ashbory,
From your profile, it looks like we're from the same era! ('70s). I may have some useful comments for you, based on my past experience. I've used GKs since the mid 70s; owned a 400B, 400rb, 800rb, and 1001rb over the years. I bought an iAMP 800 in early 2003, and just received my wt-550 a couple of days ago and gigged with it last night.
The GKs that I've owned were all excellent amps for the price. I must say, though, that I have not had a chance to try the new 1001rb II model. GKs are typically clean, punchy, and have very good headroom. I found the 1001rb had more low end and more hi-fi style high end than the 800rb. As you know, the 1001rb has more power as well. A search will provide you with tons of posts which I've read, and some of which I've contributed to. The GK posts that I've read are generally pretty accurate. GKs quality control and customer service has had some challenges in the past as those posts will indicate. I've not dealt with them lately, and they may have solved that problem.
The iAMP 800 is an awesome amp, and it has been covered thoroughly in other threads as a "search" will disclose, so I won't bother to go into detail. Basically, it has excellent tone capabilities, great punch and headroom, and is light weight relative to its power rating. This amp does everything that EA says it will do. It remains my main amp. The one thing that you cannot do with an iAMP is overdrive it classic rock tube-style. This amp will only give you a super clean tone. and I should add that Gary and Larry at EA provide excellent customer service.
I just bought the wt-550 to replace my 1001rb as a backup, and to provide me with the tube-preamp capability. Although it will serve partially as a backup, I doubt that an iAMP needs a backup. My main reason for buying this amp is that I wanted a head with a tube preamp allows me to get that fat warm tube tone, along with a slight overdriven growl if desired. Its preamp tone controls are excellent; it seems to be pretty much the same preamp as the wt-800 that I played through once. Awesome low end, punchy mids, smoth hi fi high end, but all with tube warmth. I give it high ratings for punch and headroom too. and don't forget, it's very compact (only there's no handle, I guess it's meant to be racked).
All of the amps discussed above are very good pieces of equipment. It all comes down to your personal preference in tone and what you can afford / what you want to spend. I know that it can be difficult, but the best thing for you to do is to try these amps out if at all possible. You'll be able to make a decision taht you'll be happy with. Happy amp hunting!
