What constitutes a "high output" moving coil cart?

Depends on your overall system gain, intrinsic noise floor of your electronics, typical listening level, etc. Basically LO MC is generally considered to be <0.3 mV. Most MMs deliver about 4-5 mV, I think. Many HO MC carts are spec'ed at 1.5 to 3 or 4 mV. There is a cartridge calculator at the KAB web site that will let you see the relationship between the parameters. If you have a smaller room, high pre-amp or amp gain, and/or listen at low levels, you may even be able to get into a MO MC cart.

All I can tell you is that I use a phono pre for MM or "HO MC" and my HO MC cart yields 2.5 mV. It has plenty of gain if that is any help.

I have found most HOMC cartridges specify a 2.5mV output and that makes them compatible with most MM input sections on a preamp. My Blackbird is in this category and works very well in my system through the MM input on my VAC preamp. Below this output a SUT or Head Amp could be required if you do not have a MC input in your preamp. There are a couple of lower rated cartridges (the Denon DL110 and Denon DL160 in my experience) that are rated at 1.6mV, but operated fine through my MM input. I had read that they actually put out a higher voltage than their specifications stated.

With no music playing, set your volume control to the highest position that you would normally set it at with the MM cartridges you have used. Then see how much higher you can turn it before the hiss level at the listening position becomes objectionable.

Let us know what those two settings are, in terms of clock-face positions (e.g., 12 o'clock, 3 o'clock etc). Assuming that the first of those two settings lies at or above the 12 o'clock position, if you can rotate the control perhaps 45 to 60 degrees or so higher before the hiss level becomes objectionable, and without running up against the top of the volume control's range, I suspect that you would be good to go, in terms of noise performance and gain, with any HOMC, down to perhaps 1 or 1.5 mv. If the first of those two settings lies significantly below the 12 o'clock position, a somewhat smaller amount of rotation would be equally good, as volume control adjustments become increasingly coarse at lower settings.

The smaller the difference between those two volume control positions, the closer you would want the HOMC's rated output level to be to the rated output levels of your MM's.

If you have access to a sound pressure level meter, all of this could be figured much more precisely.

Keep in mind, however, that there are issues of mechanical compatibility between cartridge and tonearm that have to be considered as well, particularly the relation between cartridge compliance and tonearm effective mass. Google those terms and you'll find lots of additional information.

Regards,
-- Al