Never Owned a Tube Amp and Want Advice

EL34 are a bit warmer than KT88, which are very slightly warmer than 6550C. But these are gross generalizations, and its a mistake to focus on the tubes. In reality the transformers are equally as important, as are the quality of caps, power supply diodes, and the overall quality of construction. In other words tube amps are just like everything else. To focus on the tubes without considering transformers is as bad as to focus on the drivers while ignoring the speaker cabinet.

Honestly, I know you want a tube amp but my advice is forget that until you're into some more tube-friendly speakers. 90dB can work, 95dB is a lot better, and 98 or more is beautiful. Because while tube watts are greater than solid state watts still you are much, much more likely to find a really good sounding tube amp in the 50-60 watts or under than 100 watts or greater.

Realize that because dB is logarithmic doubling power from 50 to 100 only gives you 3dB. My last speakers Talon Khorus at 90 dB were pushing my 50 and 60 watt tube amps hard when playing loud. But if I were to go to a 100 watt or greater amp it wouldn't be worth it for all that money (and long term tube expense) for just 3dB. You're 2dB less. You are screwed. No one else here will tell you this cold hard reality.  

With speakers 95dB or greater though, now you can take your pick of really great amps that will absolutely kill anything SS and play nice and loud, and with even better bass. You experience this you will forevermore roll your eyes at all the rubes mindlessly repeating the SS bass mantra. Right. As if.

So speakers first, then amp.

OP; what speakers will you be using?
Impedance will determine how well an amp drives the speaker; ie, a speaker with an impedance curve that stays in the 8ohm range will be easy to drive. The 88dB sensitivity may be on the low side but is less important if the speakers have an easy load to drive.
If you need high SPL’s in a large room, higher sensitivity speakers are required.
So, need to know more than the nominal impedance spec, although 50 to 60wpc may be fine.

The tube which has the most impact sonically is the gain stage input tube, often a 12AX7. Power tubes will provide the timber or tone of music; warm or neutral, bass and treble extension.
Transformers play a very important role in amp implementation. They have a sonic signature and the amount and quality of current delivered will affect speaker performance.

you are asking a question that takes a short book to properly answer

i suggest you do a search for something like ’hifi tube amp basics’ on youtube, and watch several entries that pop up - you will educate yourself faster than reading peoples' snippets here

FWIW, I have Quicksilver 60wpc monoblock tube amps and at first I thought I had to have high sensitivity speakers. I did try some at 92, 93 db and they had an easier time than speakers I tried at 87 and 88 db but I have to say I found that the Monos were perfectly fine powering speakers as low as 87db. They sounded delightful and not at all underpowered. I do suspect that a lower powered tube amp might have had more trouble, but my QS has a honking transformer and easily controls those lower db speakers — which I liked better. Impedance, as lowrider57 said is very important. I had 92 db Focals which had a bad dip in the 4 and sub-4 ohm region, and that proved a problem with the sound, in my setup.

As others here have said, this is a huge topic, and I will admit, I am a newbie. So all I am saying is that I *thought* I had to rule speakers "under 90 db" out of consideration -- and I found that proved false by actual experimentation. Also worth noting: in my space, I only need to play speakers about 8 feet away, at about 80db volume to sound good an loud.