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.