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.