Hey there Mopardave ,
I haven't heard any of the 845 amps you referenced,
but I did do a comparison with a 90+lb Cary 845 stereo amp
with my custom made transformer coupled 300b amp and,
a custom 45/2a3 stereo amp.
A little summary Using my 604G's Markwart x-overs, Similar efficiency to your M19's
and either an ARC Sp3a, or Fisher cx400 pre.
Best sound by far is the RCA45's,
followed closely by the RCA 2a3's more heft at the bottom
with the loss of a bit soundstage and detail.
followed by the 300b amp. Obviously more Balls,Good Bass,
but great mids as expected.
last was the Cary 845. Bass and output was total overkill.
Strong everywhere. But not anywhere as satisfying as the other amps.
Just for S&G I hooked up the 845 amp to my Notoriously Power hungry JBL 4430's
Just Wow! 22 Watts SET, I never heard those 4430's sound so good!
Those 22 watts sounded like 120 Watts.

BTW My amp builder won't repair Most Big High Voltage amps. Lethal Voltages.
Keep the Cats and Children away.

bobhorn