On sound quality...
The SVS new Ultra being ranked over the Fathom.The SVS being more efficient and driver's piston having to work less(less excursion)to reproduce a given frequency contributes to the better percieved SQ.
Look at the SeatonSound Submersive,uses two 15" drivers in place of one 13.5" very high excursion unit.Again the SQ would be in favor of the Submersive at high SPL. IMHO
This will be the case with any two high quality subs,the less excusion you have the better.Even with the quite advanced "linear" motors found sound quality worsens when you start to reach very high excursion.
Not that it matters that much as hearing is not linear and at very high SPL natural ear protection kicks in.
It is fun to see a beefy driver with 4" P to P excusion,but it does not add at all to sound quality,better use duals each with half the excursion.