What would the major differences be between running two 250W rms 12" subs vs
one 400W rms 12" sub. I am talking about powering it with 300W rms. Which
one would give better SQ?

Also I suppose the 300W amp will drive one sub better that two, is this
true?

I listen to rock music mostly and I need some high quality bass not that
cheap high SPL ****

I listen to rock mostly as well, and have tried various setups without
changing boxes or amps, so I can give my unbiased thoughts on this.

My subs tend to like power (350W RMS for my old 10's, my new 12's are
rated for 400w RMS). With my old setup, I had one amp driving both 10's
and it sounded good, but when I switched to one amp for each sub, it
really woke things up.

When I built the box for my 10's I bench tested it in my house, with my
Kenwood KAC-7201 amp. I tried three different configurations and I'll
report my findings.

1) L channel to L sub, R channel to right sub. Amp threw 150W RMS into
each sub and saw a load of 4 ohms on each side. I'm not going to say
whether this sounded good or bad, I'm going to use this one as a benchmark.

2) Bridged the amp, wired the subs in parallel. Amp is rated for 460W
RMS into 4 ohms and it was seeing 2 ohms, so probably somewhere over 500
W going into the pair of subs though I have no way of knowing. This
setup was noticably louder than config 1, with no drop in punch or
percieved SQ.

3) Took one sub out of config 2, so the amp was still bridged, but it
was only running into 1 sub, and it saw a 4 ohm load. This setup was
equally as loud as config 1, though not as loud as config 2. But the amp
obviously ran a lot cooler than config 2 and I imagine the amp would
last longer.

I would suggest running 1 sub closer to it's RMS limit (300 into a 400W
sub is perfect) rather than slightly underdriving 2 subs. As soon as I
get the cash I'm going to buy a second amp so I can run 460w into each
sub. Technically it may not be any louder than my current setup, but I'd
rather know that the amps weren't having to work as hard, and I believe
that with two sets of power supplies, caps, and all the other fun stuff
in an amp, it's better to have two amps running comfortably, than one
amp going crazy. Better for SQ that is...

Hope this helps,

Aaron