If Bass were going to the bench, I would start Pietrus. It does no good to go big against this Sixers team, who are suffocating the wings, and packing the paint. The C's defense is not an issue. The issue is offensively, and Philly has come in with a gameplan to force the C's to beat them with jumpers from PG, PF and C.
Ultimately though, the best solution is to keep going with Bass, hope he and KG can keep them in the games with their shooting, and hope that when it comes down to it, the C's perimeter players will pick up their game, and win some of the one on one battles they are losing right now.