Not having a Subsonic filter is certainly not a deal breaker... In a sealed box, most will never need one. In a ported box, it would depend on your tuning frequency. In an IB setup, they may help, but certainly not NEEDed.
If you feel you must have a subsonic, but the amp you like doesn't have one, simply do this..
Line Level Crossovers
Why pay $100 more for an amp just because it has one built in?