The only person I agree with is Sit -- buy preowned equipment. Remember, it's going to be used when you sell it; so it might as well be used when you buy it!
Now, bearing in mind you own a pair of Von Schweikert VR4 SE's, along with some other very sophisticated equipment (in case the previous posters forgot to to check ;-) my response to the amplifier suggestions so far would be (at best) courteous disdain!
Your reaction to hooking up your NAD is typical. We all respond favorably to changes like that -- FOR A MINUTE, because human beings are stimulation junkies. Big deal! My first suggestion would be to give that lovely Audio Research VT-100 MkII a big hug, and get some great tubes. BTW, how do you know it needs re-tubing? Is the bass getting mushy? That's usually the first sign if you're running the speakers full range and the power tubes are getting really old. If you need 6550's, the very best on the planet would be vintage Tung Sol black plates (which will cost you about a grand for 4 pairs but will be worth every penny!) or Winged "C"'s (the REAL Svetlana's ;-) which (when I had ARC amps) I bought here
http://www.conusaudio.com/ and which will cost you under $300 for 4 matched pairs. It is important to remember however, that the actual "sound" of that particular amp is more governed by the eight 6922 input-and-driver tubes. These tubes tend to last a long time though, and so people (unfortunately) don't often consider trying different ones (tube rolling) which you definitely should do, if you ever expect to tailor the sound of your amp to your liking. What 6922's are you using now? If you answer "The ones that came with the amp", then there is a LOT of room for improvement and joyful experimentation. I can say from personal experience that none of the previous amplifier suggestions can come even CLOSE to outclassing a properly set up VT100 MkII, even though I currently prefer my McIntosh ;-)
I do 'get out' often enough, thank you very much, although I tend to avoid places like Audio Advisor!
.