The beauty of a sub in a SQ install is that the sub works with rest of the components hand in hand and blends-in with the rest of the components so well that no one should be able to judge where the midbass/ woofer gave-in and the sub took over.
TRANSITION SHOULD ALWAYS BE SEAMLESS....!!!
Then comes the accuracy, the sub should be very accurate and fast enough to run with the components, if the sub is not fast enough the setup wud sound muddy.
avoid subs with very large surrounds.
avoid subs with very rigid motors.
avoid subs which cant hit 80ish hz notes accurately.
avoid subs with very high rms rating too... you dont need to power the sub too much so that they over power the components.
Powering a v high rms sub with little power will looses accuracy, if you will power it too much the bass will overcome the vocals and highs, which you dont want at all.
300 to 400 rms sub max is advisable in ur case