Ellyjr, First a caveat - I've never heard a Power Two in my home in my system. I have used and still own its predecessor, a SFS 80 as well as its larger brother the SFM160 the predecessor to the Power 3.
I used Sovtek 6550's, GE6550's, SED6550's, Sovtek KT88's, SED KT88's, Tesla EL34's as well as E34L's om this amp. It initially came with KT90's and 7308 tubes. Big bass and good lower mid-range, but a bit cold and clinical in the highs. I next went thru the EL34 group. I liked the SED's but the power was halved and I needed the whole 80wts. The Tesla's were more lively in the highs but in the long run, for me tiring.
I boiled down my selection process for power tubes to 1) SED 6550's which produced a tight/deep bass and remained linear thru the mid-range (but it was not 'warm') and slightly soft in the highs, and 2) SOVTEK KT88's which had a warm but fairly clear mid-range, was a little rolled off in the highs and the deep bass was not as 'present'. But a very nice, relaxed sound. I have found both of these tubes very rugged. The Sovteks are very cheap, BTW.
Next I played with a lot of small tubes. With the SED's I liked both the EI6DJ8's and Tesla6922's. The former are clearer and more tonally neutral - the latter were a bit softer in the highs and 'fuller' in the upper bass. Ultimately I selected the SOVTEK KT88's and used EI small tubes.
I subsequently got involved with a different amp which was close to ideal for me with SED 6550's except with my other stuff the upper mids were a bit uneven and after 2 years of playing with different types and different brands I ended up with the SOVTEK's again - same sonic signature I detected using the SFS80. Note that in a different amp the SOVTEK sonic signature was the same BUT it had a totally different effect on the amps sound, i.e. enimic bass, too soft in the highs, and a loss of a sense of dynamics.
In response to your questions - Will KT88's make my Power 2 sound warmer and smoother. Answer - Maybe warmer, maybe not smoother. Will the GL KT88's make my system warmer and smoother - maybe smoother, probably not warmer than good 6550's.
What would I do if I were you? Start with just some new production (inexpensive) small tubes and see what they do you your present system. In general the JJ's will give you a slightly warmer upper frequencies and fuller bass and the EH's will give you a fuller/deeper bass and extended highs (and in mystuff a sense of a mid-range suck out). If you can find EI's, one of my favorite tubes, you will get an excellent/neutral response that is anything but clinical or warmish.
Once you have gotten a little experience playing with the small tubes you can start considering the power tubes. Eventually you will find a combo you will prefer.
The cheap/quick solution if you don't want to go thru the process. I'd get some Sovtek KT88's and some JJ6922 and live with it for a year or two and see where that takes you.
Hope that helps a bit.
