the reason why the 305/306 series are preferred are due to their rugged built design (and since they were made in mexico unlike the newer generation). Plus they are grear entry level spl subs. But if you want quality over loudness, indeed selling both the subs would be better. In return get a JBL gto or infinity Ref 1262... the other 2 options are the JBL gt5-12 or Infinity 1250w. these are all perfect all rounder subs.
the caliber amp should be enough to run 2 speakers and 1 sub, however if u can upgrade to a pioneer 8604, it would be loads better.