Ive tried a few types of tubes in my 50 watt in order of worst to best (IMO)
6L6<5881<EL34<KT88

6L6's sound loose in the bass and overall muddy low end with an overly glassy top end, sounded harsh to my ear. (Brands tried- JJ, Mesa(sino))
5881 similar to 6L6 but tighter bass and more compressed, sounded a little grainy IMO (Brand- Tung sol)
EL34 my favorite next to the KT88, tight full bass, present mids, good attack, Bright sounding but more pleasant than the 6l6's high end.(brands- Shugaung, Winged "C" SED)
KT88 huge and full sounding, overall pretty balanced sounding, not harsh at all but also not dark. (Shugaung)

to the OP question...

Why not get both tube types?  I'm pretty happy with vintage Sylvania 6l6gc's in mine and I have a couple sweet sounding vintage pairs of el34's waiting in the wings, one Tesla, one Siemens.  The 50 watt HRM I have has plenty of both volume and girth in the tones it provides, especially through the JBL filled 2/12 and 2/15 cabs I use with it.   Cheaper and easier to obtain and tightly match pairs of tubes than quads if you want to do that.

Peace.

Cool!  Sounds by the choices of speakers that you own that you don't exactly follow the herd as much as you use your ears...   I'm curious about the Altec 417's, I have a 16 ohm 418b, basically a 15" with similar motor as 417.  Love that speaker clean, big, round, fullrange, and dirty it has a lot of bite to it, more snarl for a given amount of gain than my JBL's do, they are designed to be more smooth while playing in the dirt than their aluminum domed forbears. 

I'm guessing that you have experience with at least 40-50 watt amps based on your already owning 6l6 amps.   Personally, 50 watts is still pretty loud and proud, with sufficient headroom for my needs, but I also don't mind a bit of hair in my loud cleans according to my pick attack.  I'm glad for separate master volumes for clean and OD channels in my HRM!  It does the classic clean fender tones very well thanks to the bright, mid, and deep switches.  And you will be able to do the bias tweaking yourself after reading the amp manual, with the help of those trained ears of yours too!  I have heard that the 100 watters don't have to be very loud to sound good, my 50 watter sounds great even at small room volumes through my cabs.  i'm sure I'll love it when I decide to stick in a pair of those el34's.

Keep in mind that the standard OTS/HRM circuit might be designed for 6L6 tubes. I had the SED Winged C EL34's in my stock OTS and later bought a pair TAD 6L6WGC-STR tubes which I much prefer. If you're an EL34 fan you might like to tweak the circuit a bit. I did found the stock OTS too bright and in-your-face with the EL34's, the 6L6's are warmer and softer.

+1  The amp was not designed for that tube, there is some over capacity built into the power supply and OT, but likely 6550/kt88 is capable of pushing things a bit too far as far as plate current draw, especially at high volume settings.

I run it "full tilt" nearly everyday. I was aware there could be problems with this settup but I did it anyway. Everything is holding up very well. The transformers barely feel warm to the touch after over an hour with the master on "6" (full volume, any higher is just PT overdrive)

The amp has been settup like this for several months.

Excellent!  I like hearing from one who doesn't follow the herd, does his own science experiments, and gets good results.  I am still breaking in my 50w HRM and even after running it all night the trannies were surprisingly cool.  If they are like that running kt88's that is a huge bonus, 6550's are pretty huge sounding as you have indicated earlier.  Saweeet!