Help -- my system is too bright

Ditto the ProAc/brightness comment from Drubin. I loved them when I had them, but when getting new speakers, I wanted speakers that could make all the bells and whistles in Hi Fi...well, sound like bells and whistles.

Onhwy61 --

There's no mesh tweeter guard on the Studio 60's, but
you have a point with regard to off-axis listening. There
does seem to be less fatigue. I have been experimenting
with the toe-in. These speakers seem to sound good no
matter where you are in the room, or even whether you're
in the next, room, etc. You've piqued my interest in
the "ultrasonic ringing." This feels right. How did you find out about it? What causes it? How do I get rid
of it? In addition to other remedies, I'm thinking about some room treatments.

perhaps an RC Zoebel network installed across each of the tweeter's terminals may help, if in fact the ringing phenomenon is occuring. Dunno exactly how that could be verified, but it may be a fairly easy experiment to try out a network. I've never done one so can't explain how the RC values should be correctly calculated - member Sean may be able to assist with that; he's done this before.
Speaker cable HF rolloff can be related to cable capacitance / inductance; higher values of distributed capacitance in parallel with your load (the speaker) would somewhat attenuate the HF response, as would higher values of series inductance. MIT speaker cables may be worth trying here; they're internally networked by design, & can help to smooth out a brightness problem (it worked for me).
Regarding the 2 prong CDP AC cord: here's some ideas. If you want to experiment with a 3 prong style cord, then temporarily use a short (4" to 6" long) external handmade adapter device to convert from 2 to 3 prong IEC. During the experiment, the AC cord's shield will only go as far as the adapter, but if you like the sonic results then the IEC on the CDP can be changed to a 3 prong style, leaving the 3rd ground conductor unterminated if desired, or perhaps tied to chassis if no undesirable effects are introduced by doing so. Or the IEC on the end of the cord could be changed to fit the CDP's socket. Or the adapter harness could remain permanently if it's very short length & is made from quality conductor materials. Lots of options here. A Highwire Powerwrap helical adjustable filter installed onto the AC cord may be anoher inexpensive option to try.

The ringing is somewhat inherent in the design of their tweeter. I remember seeing evidence of it in an old Stereophile review of the Studio 20s. They showed a waterfall graph with a severe resonant mode in the 22KHz-25KHz area. It was reported as being the first breakup mode of the metal dome tweeter. When I had the 50s I had them in a system with either Rowland or Pass electronics and Discovery and Sumiko OCOS cables. With the toe-in adjusted so that I could still see a substantial portion of the inner speaker panel and a listening height several inches below the tweeter level I never noticed the resonance "problem". I've read that it's relatively easy to design a crossover that traps out the offending frequencies, but I believe Monitor Audio purposely has not taken this approach because they think it does more harm than good.

Assuming that the 60s are even better than the 50s, you've got a very fine speaker. I hope you can work out this issue.

Having an ultrasonic peak is not unique to the MA tweeter - most metal-dome tweeters have such a measureable ultrasonic resonance point, of differing magnitudes and frequencies depending on the design. It is assumed that the higher this peak can be pushed up in frequency and the lower it can be pushed down in magnitude, the better. It is also assumed that any deleterious effect this peak may have on the sound would be more caused by possible interference artifacts, reflected lower down the spectrum, than by the peak frequency itself (which is, let us remember, above the range of normal hearing). The perceived 'brightness' range actually occurs below the top octave (below 10KHz), well away from a 22KHz peak even for those few listeners who do hear up that high. With typical CD sources, the steep HF filtration usually employed by that format would tend to make tweeter resonances at or above 22KHz a less suspect issue.