The SBP is the better driver. However they model virtually on top of each other. You cannot get the IB an longer and the SBP is the replacement, so look no further. The QTc of around 0.6 is perfect IMO. I tried QTc of .7 and higher and I did not like it, too loose sounding.
Mobridge DA2 > JBL MS-8 > JL Audio HD600/4 > (3) Hybrid Audio L3SE & (2) Rainbow Cal25; JL Audio HD900/5 > (2) Morel HCW-10 & (1) AE SBP15