Simaudio has a "old standard" impedance of 47Kohms. Paralleling in the way you mention with, let's say two amps of 50ohms input impedance will halve that to 25K. This could be a problem with amps in the 10-20K range along with some (not all) tube or passive preamps that have a high output impedance but it's not generally a concern for solid state preamps. In your case, you would have the I5 amplifier section as a third part of the equation, whether it's utilized or not. (1/R= 1/R1 + 1/R2 + 1/R3) If R1,R2,R3 = 47K each, then R = 15.67K.
The way I understand you, the I5 would only be used as a preamp and the 4150 would take over as the amplifier. I don't see a problem with that. If you're considering using both the I5 and 4150 in a biamp setup along with the sub, that could get ugly for other reasons.
The way I understand you, the I5 would only be used as a preamp and the 4150 would take over as the amplifier. I don't see a problem with that. If you're considering using both the I5 and 4150 in a biamp setup along with the sub, that could get ugly for other reasons.