I use a Shadow and an Underwood. The shadow is a bit better IMHO so I keep the Underwood as a spare
There is a big difference between using a single pickup and a double like the Underwood. Some people say they use the Underwood with only one of the elements attached. I tried this and the sound changed dramatically. I couldn't get used to it, more woody, body, less sustain, so I have always been put off trying a BassMax. The Full Circle seems too much hassle as I don't have, need, or want a bridge adjuster fitted
It seems that the proximity of the pu to the strings and bridge has a big influence on sound, Realist -; FullCircle -; Bassmax. But then using two elements makes another tonal change. I have read that some use two singles and blend them with a preamp.
Recently I changed my Shadow twin element pu around, turned it the right way up, and the sound improved giving a bit less string and a bit more body to the sound. have you tried flipping your Bass max?
Not sure if any of this helps but it is worth considering trying a twin element pu if you can borrow one for a while
Adrian