dynaco st 70 what tubes to replace first

SED EL34's.Musical,liquidy.Don't get the ones that say Svetlana.The name changed to SED.$100-$120 quad.The 7199's don't make that much difference as long as they are good.You could get buy with a Sovtek 5ar4 if you don't have any Mullard's laying around.The SED EL34's are the best choice for what you want.

My vote goes to a matched Quad of EH 6CA7's in the ST70. But you should replace them all at the same time. But I also reccomend that if the mods to the input board, beefing up the B+ and power supply as well modding the bias circuit as well then you will miss all that the Stereo can deliver. I see you said rebuilt but some call putting all new parts in the amplifier rebuilding it. I think 7199's should be replaced in each rebuild, mod, new build. I don't consider that a rebuild. The problems that existed still exist. The Stereo 70 is a solidly built amplifier. If you want better detail than the tubes mentioned try the EH's. Not the Fat ones. Many don't like them because they are so fat in it's sound. If you can afford to find some good deals and try a couple different sets. You may be in for a surprise.

If the amp didn't come with a Mullard(Blackburn Plant) GZ34/5AR4 rectifier; get one(NOS). It will probably be the last one you'll ever have to buy for the unit. That and NOS 7199's(Philips ECG=best, or RCA black plate, Sylvania) will both make major improvements in your presentation. The original Genelex Gold Lion was a warm/fat sounding tube(the sound it appears you want, per your post). If the Russian copy is anything like it; they may please your ears better than the others mentioned. Get a skinny aftermarket power cord(ie: the Zu BOK or Mother fits nicely, but one of the Audioquests would work too) and you can install it through the existing hole in the chassis with a small rubber grommet. Some sources for tubes: (http://www.upscaleaudio.com/bEL-346CA7-Typesb_c_86.html) (http://www.tubeworld.com/5ar4.htm) (http://cgi.ebay.com/7199-JAN-Vacuum-Tube-Tubes-PHILIPS-NOS-USA_W0QQitemZ370295123083QQcmdZViewItemQQptZVintage_Electronics_R2?hash=item56374f2c8b)

if it sounds great..use the tubes that are in it.

I have a Will Vincent rebuilt st 70. It is able to use 6L6, kt66, EL34, or KT77 tubes.

The amp came from Will with jj kt77 tubes and it sounds incredible with those. I have since tried gold lion kt66( they sound very good but with slightly less 3d midrange), sed "winged c" new matched quad which has good bass but do not have the "magical" midrange and are so rolled off on top that it sounds like I threw a quilt over my speakers!

I have also tried a new matched quad of Electro harmonix el34's and they had a "thin" sound with little bass compared to the other tubes and had a "grainy" top end extension that was not pleasing in my amp.

All tubes were run in for at least 25-30 hrs.

The output tubes make the only major difference in sound quality or character in my amp. I am currently running a vintage 1961 blackburn mullard gz34, I have used an nos telefunken gz34 and chinese 5ar4. None of these had any effect on sound quality.

I have also changed from ge 7199's to nos rca blackplate 7199's with no audible change in sound from the amp.

I think that the speaker, amp, preamp synergy plays a major role in how different tubes will sound in your amp.
I would rather listen to my old marantz 2220b ss reciever than my dynaco with sed winged c's or electro harmonix el 34's in it. But if I switch to jj kt77's or gold lion kt66's, there is no contest. The dynaco is warm, magical,and lifelike sounding so the marantz is back collecting dust again.

My system consists of classe audio dr6 preamp, will vincent st70, quad 11L monitors. I have dialed in the sound I like and am very happy with my system now.

Good Luck and enjoy your tube rolling.