Sounding "good" is a relative thing. Urei 813s can be an effective control room monitor. Some of their characteristics encourage you to balance tracks carefully. Others, prevent you from filling the midrange with spectrum overlapping synth / vocal pads. None of this has to do with sounding "good". It has more to do with modulation distortion, energy smear and sub overlap, none of which seem like a good idea, until you mix a record on exotics and find it unlistenable to the public.
I have been building "hot roded" UREI crossovers since 1984 when Secret Sound sent me a defective pair and I would not let them leave with the crap inside. (I think FM measured a pair at Grandpa Studios with good comment). With all the attention paid to Time Alignment in this design somehow they missed the Df of the electrolytic caps used.
Soffit mounting is a must. Subs must be on top. A modern amp with a damping factor over 80, slew rate over 20VuSec and power of at least 400 watts into 4 ohms. Bridged Bryston 4Bs are a good choice. Don't use a Mac with an output autoformer (2300, 2500 etc). Avoid 1/3 octave EQs, use a parametric if you must.
dbx's DRIVERACK offers better driver protection than the backup lamp off a '56 Chevy, real time alignment settings with EQ for each driver plus other improvements. Don't get me wrong, I like building the passive crossovers but I think dbx is just a software update or two from the better mousetrap.
Last, these are 20+ years old and studio work is hard on speakers. Get them reconed by a first class shop. Sometimes even the magnets need to be recharged.
If you maintain your monitors, there is no reason 813s cannot continue to provide useful service.
Tom Maguire
TMI Engineering