Tough question to nail down for you.
Scott, in it's day was considered good equipment not off the wall great but a good value and a good product. As for HeathKit, I do not remember them being considered high end but more of an affordable option for kit builders and at that time so was Hafler and others which is not to say they were bad at all. Having said that line of conconformist crap, if it is possible, see if you can make arrangements with a local dealer to demo some tube equipment for the week or just a weekend. Personally, I think for the biggest "bang" improvement I would try a tube preamp first and see if you like the sound. After that then it is small steps. I would say tube equipment has the largest amount of variance with respect to sound versus solid state but then again I may get shot for that statement too
You may find that it is not that great of an improvement and prefer to stay with solidstate (SS) which is fine. Tubes can get quite expensive and that may too be prohibitive.
As Roberto always says, and we all steal from him, "trust your ears"