That Eico HF-81 basically has the HF-85 preamp integrated into it, if you took a HF-85 and did a complete rebuild with
the upgraded tone circuit boards and replaced that crappy stock volume pot with a ALPs blue velvet and replaced film
and electrolytic capacitors along with most the resisters with carbon film, upgrade the wiring with shielded wires in
many places and of coarse replace those crappy RCA jacks you would have a fine preamp and when connected to those
Heathkit UA-1 amps it might just tip the scales and come out to be the better performer.
I hope one day to have the opportunity to do just that. As I noted previously, I really wanted a restored 85, but they are far and few between. At least where I look.