Hmmm, interesting. I've got the OEM sides/rears in parallel (1 ohm) running off one channel pair on the HD900/5. The fronts (4 ohm) are driven by the HD600/4. With gains set to minimum on "low" (2V) the rears are a few dB louder than the front doors. Theoretically, the max output of the HD900/5 at 1 ohm is 75W and that of the HD600/4 at 4 ohm 150W, so the fronts should be a lot louder, but they are not. I'm not worried about it though since I can raise the gains on the front channels to compensate.
The "low/High" input switch discussion though has got me thinking. I've had a problem with noise on the HD900/5 amp. When I switch from low to high, the noise goes away but of course the output level gets reduced and of I raise the gain to compensate, the noise comes right back. I was thinking, since the MS-8's speaker level outputs are of very good quality, what if I used those instead of the low level RCAs? Any potential downsides to doing that? This may be a good solution since it would kill 2 birds with one stone - getting rid of the engine noise AND utilizing the reportedly superior input option on the HD amps.
Mobridge DA2 > JBL MS-8 > ARC XDi 600/4 > JBL Gti 408 Mids & Vifa OT19 Tweeters; PG Ti2 1600.5 > Morel HCW-10 & IDMax 15