Are looking for the items themselves or opinions on them?
I had a Randall RM100 and thought it was a great design that operated well and the modules sounded great! I have my favorites, but they all sounded really good (except one of their earlier Recto modules - it was undefined mush)
Egnator makes a similar preamp that can use either Egnator dual mode modules or the Randal Modules. Egnator designed Randall's modular systems.
MP1 is an old staple, the RM4 will offer much more flexible options that do sound as good as a high quaility tube head when paired with a good tube amp.