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

I think either a Y adapter or a dual output block available from AudioQuest is your best answer. Unless you have XLR then the Y is all there that I know of.

a Y connector is fine. Another alternative is if your Sub has hi level inputs (as most subs do) you can connect it to your amp using those. Your sub may also have pre-outs that incorporate the Sub's x-over, and you can connect your 4150se using those outputs.

Or you can have a full length set of y cables built. I know audioquest has done them in the past; no reason why other manufacturers could not, if they do any custom work.

I just had a set of dual balanced cables made to bi-amp my speakers. Try Frank at Signal Cable, he made them for me.

you can add another set of main outputs on your preamp, if there is room.

I have a custom set of Y connectors [ 1 male to two females] that Audioquest made for me with their Emerald cable when I had this problem. Contact me if interested.

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.

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.

Most preamps that have two sets of output jacks simply have them wired together directly behind the rear panel. There are some that have separate buffers (output amplifier stages) for the two sets of jacks, but they are in the minority.

Electrically there is no difference between using a y-connector on a single set of output jacks, and connecting two load devices to two sets of output jacks that are wired together internally. There may or may not be subtle sonic differences between those two situations due to cable and connector effects.

But in BOTH cases (using a y-connector, and connecting two loads to two sets of output jacks that are internally wired together), you are correct that the preamp output will see a load impedance which is divided by two (assuming both load devices have the same input impedance). That may or may not be significant depending on the output impedance of the preamp (the lower the better), the input impedance of the load devices (the higher the better), and possibly the current drive capability of the preamp (although I don't think that is likely to be a factor in typical cases).

For a typical situation of an active preamp that has an output impedance of say a few hundred ohms, and the power amps having input impedances of 50K or 100K ohms, the 25K or more combined input impedance of the loads will still be far greater than the output impedance of the preamp, and hence still not a significant load.

There is one other factor that needs to be considered. If the preamp output impedance is highish, and/or the interconnect cables are either long or high capacitance or both, then the upper treble could be rolled off somewhat. That is also true in the case of a single interconnect between a preamp and a single power amp, but adding the capacitance of a second cable can potentially worsen the effect. For a typical situation, though, involving one or two meter interconnects that have low or moderate capacitance, and a preamp with reasonably low output impedance, I wouldn't expect this effect to be significant.

Regards,
-- Al