Your opinions on what speakers to go with?

I pretty much disagree with Shadorne. Nice system! I owned 150s. It should be in Ultralinear and you should be using KT 88s. Also I found tube bias needed to be adjusted regularly. You could have some dead (or near dead) tubes.

Power cords, interconnects, and speaker cables all make a difference. I really like Morrow Audio MAP2 power cords, Either PS Audio Transcendent silver interconnects or DH Labs Revelation. Get XLR if the preamp has XLR I/Os, if not replace it with a differentially balanced preamp! Ditto the Consonance. The Raysonic 165 is killer and the balanced tube outs are so good you can eliminate the preamp all together! Clear Day solid core silver shotgun speaker cables. If after doing this you still have speaker issues look at Usher, Triangle, Reference 3A...

Of course, if this is something you are only experiencing recently then Tweak could be quite correct - you may have a tube issue. Another challenge could be microphonics when you play loud. Pink Floyd's studio have their tubes in an entirely separate room to avoid the microphonic problems at higher volume levels. I seriously doubt if cables/interconnects can cure a problem that only appears at higher volumes.

My advice was based on your stated interest in trance/dance genre and "like to listen loud". Check out this setup from a guy who mixes for Kaskade and other DJ's for club music.

I THINK ONCE YOU HAVE CHECHED YOUR TUBES YOU COULD UPGRADE YOUR SPEAKERS... CONSIDER USHER AUDIO 6 SERIES AND TYLER AUDIO SPEAKERS... BOTH WILL NOT BREAK UP WITH A LITTLE VOLUME. DEPENDING ON ROOM SIZE WILL DETERMINE WHICH MODEL IN EACH LINE WILL WORK BEST.

I had a pair of Monitor Audio silver s8's, which I believe were the british made predecessor of your RS8s. I recently dumped them off for a pair of Quad 22L2s. I can't honestly say I had any problems with distortion at moderate listening levels, but I have all solid state gear (Musical Fidelity A308 Integrated and a benchmark dac1).

The MAs have an abundance of low end base given the 3 6 1/2" drivers and dual rear ports. My complaint with them was boomy low frequency and shrill high frequency with the metallic dome tweeters at increased volume, so that may be the muddiness you're hearing (not really distortion, just poor imaging due to an overload of low end response). I agree though that power conditioning, better power cables and solid interconnects can clean up the sound quite a bit. You could probably get away with pure silver since you're using a tube setup (I found I can't as it's too much HF emphasis). I'll be the first to admit I'm not an expert (nor even close), but I've learned some through the growing pains of system upgrades and dealing with MAs.

The Quads throw out a much better sound stage and better imaging....at the expense of low end frequency response of course (one less 6 1/2" driver and one less rear port).

I strongly suggest you check out Digital Phase AP-4's.
They are made to order for your requirements.