The EEB mods were like items on a menu, I just tossed out an old catalog of theirs a while back, so I'm going by memory here, but I think it consisted of an alignment (Some of them are off by a huge amount out of the box), replacement of some audio components, and possibly a filter swap. The R71a is a pretty poor sounding receiver, in AM mode, even with mods done on it. I've had two of them, and they both sounded "rough".
I bought my Yaesu FRG-8800 from EEB in 1985. Due to a screw up on their part, if I had been patient and not called to see what was taking so long, I would have received an R71a instead, with the filter change, etc done, for the price of an aligned FRG-8800! The FRG had a lot better audio than an R71A, but filter wise, it was pretty bad, and soon I was looking for an upgraded receiver for RTTY, and I made the mistake of going to Universal in Reynoldsburg, and playing with the JRC NRD-515 they had on display. Soon I had one, a Gilfer modded one, and 23 years later, I still have it.