Tube issues, 12at7 affect KT88?

Heyraz. May I suggest you replace the board in it and there by replacing the 7199 as well. Both the 7199 and the 6AN8 used in the MKIII are not very reliable. That tube is a medium mu triode/sharp cut off pentode. Although a good idea at the time of the design, it's hardly enough tube for the job. In fact after about 6 months or a years worth of wear on the tube and one half of the output waveform will prematurely clip. You will never get full output and distortion products will go up from the premature clipping. Also, hum is a distinct possibility and the culprit is that 7199/6AN8! Since the amp was designed around the RCA version of that tube, it's the only one that seems to work the best and those are very hard to come by. There are a number of people making upgrade circuit boards and power supplies. I would however keep the GZ-34/5AR4 tube and not replace that with a solid state device. Also, be careful with the circuit board upgrades as you can run the risk of overheating the power transformer attempting to power too many tubes.

The Amp is stuffed away in the box so I'm really not sure what version it is, but if I'm reading into your message correctly, the DynaKIT is based on the DynaCO MKIII. Is that correct? I never realized there was a difference until now. The 7199's say Dynaco Made in USA. Also, thanks for the advice regarding overheating the of power transformer, that's the first time I've ever heard that.
What driver board would you recommend?

That's a good question. I haven't really looked into them. Find out what your options are and let me know. Dynakit's were available for most Dynaco products including the FM3 tuner. If you use more tubes on a new mother board, depending on the tubes used to replace the 7199/6AN8, the filaments of those tubes will draw more heater current. Hopefully it's close to what the 7199 drew. Search the internet for circuit board updates. I believe Joe Curcio makes one. He designed some of the circuits for Sonic Frontiers.

Curcio Audio Engineering has too many (well 3) options to go into on this thread. Basically, it's stick with the 7199's if you are restoring, an all triode model fully direct coupled, or all triode CCDA. Non of which makes any sense to me at this juncture in time. I assumed the other versions came about because the 7199's went out of production and became scarce and drove up the price ( I know an adapter is available). Some threads I've read prefer that ol' Dynaco sound, others, well you get the picture. I planned on researching and learning this craft a bit more before I began the project (I'm more of a solid state guy right now).
If it's not to much trouble, could you explain the differences mentioned above or do you know of a site with that info? I've been torn between restoring or rebuilding, but if it's a MKIII rather than the original 70, I'd probably lean more towards a rebuild using state of the art components for a more controlled sound.
Thanks

Well you do have to draw the line somewhere. When I do a mod on someones preamp or amp I take a long hard look at what was the intent was of the designer of the circuit. In the case of Dynaco, there were always cost considerations. This was the stereo for people who couldn't afford McIntosh or Marantz, or wanted to build it themselves. Heath kits were also popular. Obviously with the ST-70 or MKIII I would never replace the transformer unless it was damaged. BTW on the MKIII's I had heard that the older gray painted transformers were better then the newer blacks ones but I was unable to prove if that was true or not. There are other sites with other circuit board replacements. I would check them all out. I have absolutely no problem with an all triode tube input and driver section. If it's something like a 12AU7 coupled to a 12BH7 that works just fine.