Audio research M 300 mkII vs Classic 150 triode,

Jag you are welcome to listen to my arc classic 150's.
I am 50 miles nothwest of Philadelphia. I have found them to be very musical amps.
I owned Conrad Johnson before these and have found these gave me a sense of more space and air. I also found the midrange to sound more lifelike.
I have seen them sold here at a great price. I don't believe you can go wrong with either of the two amps you are considering.

I actually had my M300 Mk IIs converted to Classic 150s by ARC, and noticed no loss of high frequency extension (if anything, I recall it being better, more refined after the change) or bass extension. Would not have gone back; the Classic 150s were a much better amplifier, more refined compared to the brute force of the M300s, with the same slam and power. Was driving Duntech Princesses at the time, a fairly easy load that went down to about 27 hz in-room, but I'd expect it to do well with the Monoliths as well. If you can take up Yeigh on his generous offer you should (nothing beats hearing with your own ears), but I think you'd like the Classic 150s, particularly on jazz.

Anyone know how M300 vs Classic 150 taste test might do with Magneplanar Tympani Iva's?

Or biamped with a Levinson 23.5?

Cw, my recollection on the Maggies is that they need a lot of power to sound their best. If that's right, you might be better served by the M300s, but I'm only guessing here.

You could buy the M300s, and later have them switched to triode operation by ARC. I had the M300s in triode driving the upper panels of my IRS Betas and they had more than enough power and sounded great!