Head amp's are hen's teeth, they make SUTs look positively common - in terms of availability you have one choice s/h, but also available new - Rothwell. I found both their SUT and head amp' good but rather pedestrian, but each unto their own.
Less common s/h but available new is the Graham Slee. I preferred the Rothwell overall.
There are numerous other SUTs around of various ages, but almost no other head amp's that won't take some considerable searching and/or money to find. Both SUTs and HAs come in varying degrees of adjustability, from none upwards.
I am actually currently using a Yamaha head amp', suggested by someone here. I am not looking to replace it. It has a high and low MC input setting, plus a MM setting, which I have no idea as to what it does, but probably a straight-through connection.
Current cart's - Lyra Delos and Transfiguration Spirit.