I had an 804 and now an 805 and I’ve been using various flats on with various results. Darker sounding flats sound better on this style of instrument in my opinion, the piezo can get really quacky with the tone turned up and brighter flats sound even more abrasive IMO. The best tone for the dough came from GHS Precision flats, they don’t have any of the buzzsaw like mwah that the stock chromes can’t help but do.
I believe the GHS Tapes will be my next attempt as the GHS flats were among my favorites on the bass. I have the Labella Jameson set on mine now and they are the stiffest strings known to man, definitely a far cry from the Thomastic Flats they replaced. The Thomastic small A string would move around in the nut and caused an incessant buzz. The strings were supple and easy to play though. My bass guitar experience with tapes is that they are more supple to play on than average flats and also have a larger gauge which feels more “right” on the UB. It’s definitely a journey worth taking, once you find the right strings the bass becomes a true joy to play. Pound for pound the most fun bass I own. Try it with an octave pedal! The piezo tracks every note, even on the B string.