Like many, I participate in various forums on the ‘Net, and I came across a plug for a fascinating article written in 1998 talking about devices that employ tubes – specifically amps and hi-end audio – and does a very good job of explaining why people gravitate towards tubed devices. Here’s the link to the article. It’s long, but worth it!
http://spectrum.ieee.org/consumer-electronics/audiovideo/the-cool-sound-of-tubes/0
Guitar Gear 
Interesting read (half way through). So far it hasn’t gone into the ability to dial in the sound with tube types and values. Adjusting the tone of the amp overall is possible with pre amp or even power amp tubes.. One of my Rivera’s went from EL 34’s to E 34L’s to KT77’s and two days ago to 6L6’s. I prefer the 6L6’s but the E34L’s are a close second. Were the tubes changed due to wear..? Not even once.. There was a grainyness to the upper mids with my V30 loaded cab, albeit slight, still an annoyance. Now it’s gone.
My JCM 800 has NOS 6550’s with two high gain JJ’s and an older low gain PI tube, why? After trying many combinations that one sounded best to me.
Chris, have you tried KT-66’s? Nice, bright tone, and don’t overly compress when pushed hard.
For many years in my early playing era I purposely would seek other tech over tubes having been schooled by my brother’s opinion on such “low tech and simple circuits”. Later on after a good many years of major rack mount systems and high tech rigs, I, by chance, happened to get my first “twin” amp 2×12 from Carvin using 5881s, something interesting was happening there, although I have to admit for me it is the 4×12 cab, have to have it, sounds better regardless.
Once I eventually got my Marshall half stack there was no turning back. The EL34s just created something special for me. Sure the distortion quality of the tubes was unique and cool and not so sterile and tight, but what I got from that experience was how much better my beloved pedal board sounded and felt through just clean tubes. To this day (many decades) I run an extensive, rather cool, and diverse pedalboard through 90% of the time a clean vibrant tube channel. I find the issue now to be between the amazing EL34Bs I came across from Tung Sol and the 5881s, right now I am rebiasing and switching over to the 5881s and I run JAN Phillips 5751 preamp tubes w less gain in my amazing versitle Carvin V3 head w the vintage 4×12 cab of theirs. Really the best tones I have ever gotten out of any rig in my many years of playing and searching for the grail. As much as I would love to escape the old school tech of tubes and their easy to wear out properties modeling has not come close.
I often try to explain the difference in geometric electronic visualization. Digital no matter the vast amounts of 10s of thousands or millions of quantizing sample rates cannot make a curve round like the analog of tubes. The digital no matter how advanced “steps” around the round curve of the tube and even if you get the feel and response solved and the compression quality you still have those tiny, tiny, little notches on the curve where the digital cannot quite get the full round, it is close and gets better all the time but it is the inherent issue of digital stepping and analog round curve, those missing little pieces are what makes the old school have that “tone” which is still not quite there.