Tube replacement for Dynaco ST 70

I just purchased a highly modified Dynaco St 70 and it came with EH EL 34 and I would lke to replace them since I feel they are a liitle thin and veiled on the upper midrange and lower treble regions where the voices are produced. Any suggestions on replaceing them with tubes that will open up and unveil the voices. I can uses 6L6 or KT 66.
Make sure your highly modified Stereo 70 does not include a solid state rectifier with a lot of filter capacitance. Unless there is some kind of ramp up circuit, that's a sure way to damage the secondary side of the power transformer in a hurry. I already have one on the floor of my office where someone 'modified' their amp only to short out the winding of the transformer. Regarding tubes, I would try a different EL34. The EH's might be the problem. Just understand that if the tubes, or the updates are new, that will take time to break in. Normally output tubes can take 20-30 hrs. to break in. That does not mean to leave the amp on 20-30 hrs. Just keep listening to it when you can, and the sound will slowly change as things break in.
A few years back, I modded a ST-70 and used 6L6GCs for the output. However, I had to replace the bias potentiometer in order to have enough range to accommodate these tubes.

Triode Electronics in Chicago has some some driver upgrade boards available that include excellent instructions for a variety of modification options.
I have the JJ E34L's in mine and have been very happy with them.
I feel that the EH tubes are really good in guitar amps, but prefer the JJ in the Dynaco. notice that these are E 34 L not EL 34- so they draw a touch more current. Mine started sounding pretty good after about 10 hrs. They didn't sound BAD at first , just started to open up after ten hours.
If you want to go the 6l6 route, I would definitely go with KT-66's. The KT designation specifically refers to "Kinkless tetrode" which means that they were designed to remove a "kink" that was present in the frequency range of the 6l6.
First of all, could you tell us about the modifications? I've personally used several different mods of the ST-70 with a number of different tubes. I've recently used Siemens, pre-Sensor Svetlanas, Winged C's, Shuguang Treasures, RCA's, Mullards, and the reissue Tung-Sols. Mullards are usually my favorites, but good matched pairs/quad are just skyrocketing in price. I'm also a big fan of the Siemens; they seem to be a little more neutral in the presentation than the Mullards. I might say that the Shuguang Treasures are the best of the bunch right now. If $200+ for a quad, then I would suggest getting the Winged C's. They're essentially the same as the pre-Sensor Svetlanas which have a very similar presentation to the Mullards.
JJ E34L as suggested above are a well extended and well well balanaced EL34 type that works well in the Dynaco. They can be had for a very good price from I like the blue glass versions . I have been using them for years.
I own one of Will Vincent "Audiospec" Stereo 70's.
Recently upgraded the output tubes to the Gold Lion KT66
and have not looked at other EL34 tubes. The soundstage just opened up. With this amp the improvement is huge.
All the best....
I realized I only listed EL34's. If your amp will accommodate them with the mods, the Gold Lion KT-66 reissues are outstanding - probably the best bang for the buck and one of the best 2 or 3 options available.
Firstly, I would encourage you to contact Will Vincent himself.

But my understanding is that his amps are incredibly close and true to the original Dynaco ST70. That being the case, this should not be considered a heavily modified unit.

Most would consider the EL34 more open than the 6L6, as opposed to veiled. Though as you also mentioned thin sound, if you seek more richness and body, you might want to give some consideration to the 6L6.

I would ask what 7199 tubes you are using in your unit? With the right tubes, powering an agreeable speaker, the only cause left for not finding happiness in a Dynaco ST70 would be a difficult room.
Can anyone comment on the gold Lion KT77 as a replacement for the EH EL 34 tubes. Upscale audio is recommending it
I usually trust Upscale's recommendations. I've only heard their KT-66, but I've heard nothing but good things about the 77 as well.
Exocet 954
I have Will's mod also and just got the KT 66. Can u tell me what the basis setting should be on them
Will Vincent recommends 1.40mV DC meter for KT66 and EL34 tubes. Just read your post today. Sorry for the late respond.