How to hook up 2 amps from one set of pre outs?

I always assumed the signal impedence would be altered tying in a second component to a single set of outputs (signal devided in half). I also assume you could add another main out and tie that into circuit (rather than simply pig tailing 2nd out) to maintain signal integrity. Perhaps I'm wrong, some more technically savvy member should know.

Why not run the sub w/ speaker cable instead of rca?

Not sure if your amp has two sets of binding posts, but if it does not, simply run two sets of speaker cable... One set going to the mains, the other set going to the sub. This way, you do not have to worry about Y connectors or possible problems w/ said configuration.

IMO, i've found connecting a sub this way reaps better sonics than using RCA cables, although i do realize that every system is unique in its idiosynchrasies.

regarding what SNS said, I have heard the same. I was in a similar situation at one point and every knowledgable person I asked told me splitting the pre-out was a no-no.

Just my 2cents.

I was alternating using two Meridian 605s a channel on my Duetta Sigs and using one of them and there was no difference using the Y connectors I could hear. The speakers sounded better when I was using 2 as you would expect with bi amping.

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.