Output tube types - need some opinions

Since you've already worked with pairs of 300B's I suggest building some amps with a pair of 300B's in PP. This configuration will give you about 25 watts of triode power! Using the common tetrode/pentode types (6L6, EL34, 6550, KT88 ...) requires feedback for low distortion/linearity. Then problems arise with instability due to feedback. Stick to triodes! Get the best output trannies you can afford - like Tango/Tamura. The output iron is the limiting factor in any tube amp's sound quality - much more so than the brand of the output tubes. 

I recall a magazine article from years back about building a PP 300B amp using output trannies from a HK Citation II. It might have been in Vacuum Tube Valley. This would be worth checking out! It was an impressive article!

For this next amp, I'm hoping for more headroom (at least 30W), a bit better macro-dynamics, and maybe a touch less warmth. 

I second @jasonbourne52 's opinion. I've seen PP 300b amps that make about 30-35 Watts.

I listen mostly to jazz, folk, electronica, classic rock, and enjoy vocals. Excellent vocal performance is probably most important to me, but nice soundstage and decent imaging is also a priority. I use powered subs to cover the bottom couple octaves, so deep bass performance isn't too important, but good midbass performance is. My high frequency hearing isn't what it used to be, so extension out to 20khz and beyond will be lost on me. 

A good output transformer that can do 35 Watts should have 100KHz bandwidth. I would spec the transformer to go to at least 10Hz despite the fact that you are using subs. 5Hz is better. If you find a way to make an amplifier circuit favor a certain genre, please publish details on how that is done- no-one has figured that one out so far 😉

Depending on your power requirements, any pushpull from a 300B, 2a3 or 45 tube would be a worthwhile project (I particularly like the 45, but, it is very low in output).  Spend all that you reasonably can on the output transformers.  I like vintage transformers such as Accrosound TO 330 or Chicago transformers.

For pentode/tetrode tubes, I am a big fan of 6L6 and KT66 tube.  Even more exotic would be the Western Electric tube that is close to the 6L6--350B, which is a really nice sounding, albeit expensive, tube.  My own pushpull amp runs the Western Electric 349 tube (very low output, even more expensive than a 350B).  Again, the output transformer is very important.  The vintage ones I like were those from RCA 6L6 amps and from Northern Electric amps.  The Western Electric 171C transformer, despite its limited bandwidth, is extraordinarily nice sounding with these tube types, but it is quite hard to and pricey.  

I have not heard any amp built with KT120 or KT150 tubes that I really liked--somewhat hard and brittle sounding.  I feel the same way about most KT88 amps as well.  I actually prefer most "budget" El84 amps to those using the more powerful alternatives.

By the way, I really like your build projects posted on the internet.  I am particularly impressed with the way you make each project your own, in terms of design and execution, and the build itself is meticulous, and beautiful in appearance.

A classic EL-34 is a good starting place, then maybe retune for 6L6.