Signal tube versus power tube, sound impact?

Lot's of good suggestions here, although overall maybe a bit confusing/overwhelming (no surprise).

Personally I've had good outcomes with the front (small) then back (power) strategy, other than one KT150 based amp as there are no other options besides Tung-Sol. In this case replacing the stock JJ pre-amp tubes with some NOS Telefunken Diamond Mark Red Tips worked magic. 

I have not been impressed with GL KT88 tubes vs. JJ. I returned the GLs. This was in my first tube integrated - a LineMagnetic.

A trusted source is important and I have mostly used Brent Jesse at audiotubes.com. Everything he has is listed on his website, and he gives good advice without being pushy or overselling, and has a very reasonable return policy.  

Although expensive, if you want to warm things up, the NOS Mullards are pretty special and currently in my tube DAC - however, and this is where Brent Jesse comes in - I have some 1950's GE long plates (branded as Wurlitzer) that I rotate in and out of the DAC that sound remarkably good in the DAC at 1/4 the price.

Expensive? Maybe, but I’m some cases how would you compare as he likely has inventory that no one else has?

I have returned (exchanged) 1 set with no issues.

The way I look at it is value for money: good advice, no concerns about fakes or items not as described etc.

having said all that there are several other very reputable dealers out there. Good luck and have fun!