As a general rule how much less output should a neck pup have compared to the bridge? More specifically I have a Dsonic bridge (output 390), how low can I go with the neck pup output before it will be hard to balance the volumes?
really depends on the bridge model you're using...
...I also have a D-Sonic on my RG550 and have a Breed Neck for it, those 2 seem to balance each other(390mV & 330mV respectively) volume-wise; on the other hand, my JEM777VBK has the stock Paf Pro and an Evo (<300mV & 420mV) and those seem to even out pretty well. My last experience was matching a D-Activator X with a Paf Pro (500mV & 300mV) and I had a REALLY hard time matching the volume level of the Paf Pro to the D-X.
Essentially, the general rule of thumb for HH/HSH setups, bridge pups are mounted much higher than the neck pup But so far, experience tells me 60mV – <150mV difference would be a safe bet...
You don't need that much output in the neck anyway, the strings vibrate there the most. I used set up a guitar with an air classic neck with an evolution in the bridge and it works fine although I don't think they are tonally fitted for each other but in terms of output they work fine.
I have a D-sonic(390mv) in the bridge and a Fastrack 1 (200mv) in the neck those seemed very mismatched in the begining but I just adjusted the hieght of my bridge pickup a bit and it evened out. Now they aren't completely equal now, close, but not perfect. Sometimes you have to accept that based on what two (or three in my case) pups you pick and your guitars configuration.
I went ahead and ordered the Tone zone S for the neck. then output is listed as 300 vs my 390 dsonic bridge. I know the TZ is typically a bridge pup, but I really liked the warm sounding lead tone in the audio clip. I'll give it a try anyway. I'm also thinking of getting a true velvet for the middle.