The TU-8200 is designed for high efficient speakers, especially horn-loaded, and I have only been pairing it with Spendor BC1, Rogers LS3/5a(11 Ohms) and BOSE 601 Mark II. Technically you can only describe it as under power for those speakers.

Having said that I was happy with result. It was more than enough power to play with acoustic stuff. It performs adequate with small chamber music though it could struggle with orchestral big piece.

As for the tubes choice the original 6L6 from EH are fine but nothing to shout about.

I have then bought the followings and also preferred order in rating:

1. A pair of new Gold Lion KT88
Verdit: extremely musical and detailed performance, very smooth presentation.

2. A pair of SVETLANA 'WINGED C' EL34
Verdit: strong pace and very good bass and at the same time good imagery.

3. 2 pairs of Svetlana 6550C
Verdit: very good performance with vocal and low voulme listening.

4. A pair of Chelmer Premium EL34(rebranded Sovtek EL34Gs)
Verdit: perhaps just a tag better performance than the original tubes.

And the rest would lie on par with original as they all are Russian tubes
5. A pair of MESA BOOGIE 6L6GT
6. A pair of SOVTEK 6L6GC
7. A pair of Chelmer 6L6GC

I am using this amp as well and I have used all the specified types of tube(6L6GC, KT88, 6550C and EL34).

I have not tried the KT-150 due to the following information from the net:

"Because each KT150 consumes 2.0A of filament current (more than the KT88 and 6550), we recommend contacting your equipment manufacturer to insure your power supply is capable of powering this tube."

As such I would do with much caution.

Here is mine in action.