How to fix my detailed, accurate but BRIGHT system

Sometimes a leaner bass may also manifest itself as a more forward overall sound, bringing into sharper focus other tonal characteristics.

Experiment with a little larger Toe-out on the speakers
This will also create a wider sound stage.

Also try moving the speakers back a little towards the wall / corner.

Careful speaker positioning can do wonders, and IMHO is one of the most neglected solutions...

accepting the fact that room treatments are out of order (which i totally respect), i do think cables could help a bit.

if you could ever audition a set of van den hul carbon cables (rca versions are: "the first", "the first ultimate", or my favorite "the first metal screen" {they are the most hum resistant of the rca options, which doesn't usually matter except in phono connections in my system}; and balanced is "the second"), you may be pleasantly surprised at how they tame a bright system. in some cases too much, but in your room maybe they would be just right. in my experience they don't so much roll off the high end as recess it, which is what you're looking to do it sounds.

a nice all-copper speaker cable would be a good idea as well. as WAF is an issue here, anti-cables could be a good fit visually, they're tiny.

and if you're still using stock power cords on your components, a $50 power cord from, say MAC, might make a noticeable difference in adding bottom end to fill out the sound.

so in a perfect world, you could find some used vdh firsts for around $150, try some anti cable speaker cables for around $75, and try a MAC power cable for $50-$75 and then you'd know if cables can help you out...

good luck, though, i'm with you, i HATE brightness. my favorite tv program is rugby matches from new zealand and the sound quality is so bad - worse even than the sound from any other country's feeds - that i have to turn off my system and listen to the tv speakers. annoying.

Forget cables - just get a PEQ and cut at 2Khz around 3 to 6 db and you'll be a happy camper. (Your room is too bright, your music is probably compressed and harsh sounding etc. - never mind the reason, most harshness comes out around 2 Khz - if you simply reduce the 1 to 3Khz band it will be more comfortable for you)

(Oh and BTW before anyone panics about the irreparable damage that might do to precious music, please everyone educate yourself about how recordings are made in the first place - all kinds of EQ are applied and if you have a good system and ear you can actually hear it yourself on most albums - often each track is EQ'd differently - worse some tracks have EQ changing during the track - all audible if you know what to listen for. So a decent PEQ won't "ruin" it for you)

It is an expensive fix but if you use Cardas Golden reference ICs and the best Cardas or all copper speaker wire you can afford,the sound will warm up a great deal. No need to change decor. I second the liberal use of dampening stands platforms etc.It won't be as sure tonal change like the best Cardas. It brought the bass into better focus where I have used it.
According to the the numbers, I think it is in fact a very difficult load. I respect Al Marg's math but rules of audio lore are not so easily dismissed. We all know that using SS with an increasing impedance SPLs go down. Beyond the sound levels and more importantly the quality of the sound changes dramatically in real life. Tubes like a higher impedance and I proved this to myself comparing a SS Mac with a tube amp of similar wattage driving c. 1960 16 Ohm JBLs.
With the Butler, my understanding is that most of the power comes from supercharging the 300B with SS bipolars. The natural power dissipation from a 300B is 8-10 watts.

Warning what follows you may find funny or just idiotic read at your own risk.
Finally as a bit of humor take a sheet of Dynamat and carefully cut strips of it to apply to the outside of your tweeter frame only. Start with an X then add more strips as needed until almost covering the tweeterif need be. It is crucial that you use a cage to cover the cones themselves. That stuff won't come of very easily so only apply when completely sober and but frustrated at wits end. Lead tape will work as well and you can get lead sheet without adhesive. With the no adhesive sheet you can paste it up using a very small amount of green damping glue.
Or-Keep the rig exactly as it is but try the trendy Victorian bordello look. It should be dank dark reek with coal candle and cigar smoke (cuban please,) You will have to use a crystal chandleer but be carful and try to get one the scaters sound up. The room must be repleat with thick maroon colored velvet drapes, thick wool plush cut carpets to match and scatter some deer and moose heads on the walls for diffraction. A few heavily gilded ornately framed portraits of well known patrons in real cracked oil paint help as well. You can always find authentic large gilded framed 19th century oil paintings at garage sales and such.