Welcome to the forum!
I'd like to ask how you want to handle bass management, or the frequencies played by the sub and those played by the speakers. As you noted, your Rotel RC-1580 doesn't handle bass management. Do you want to:
1. Run all of the speakers connected to your power amps full range (over all frequencies), and have the sub double up on the lower frequencies?
2. Run two of the speakers full range, and have two of the speakers only play frequencies higher than that on the sub?
3. Have the sub handle all the low frequency content and have the four speakers only handle frequencies higher than those played by the sub?
Your original question leads me to think that you want #1, but I want to verify that. If using the power amp loop out isn't the right way to do it, you can always just use two RCA splitters (one on a left output from the pre-amp, and one on a right output) and run cables to both power amps from a single set of outputs on the pre-amp. That will work just fine.
If I understand your Velodyne manual correctly, using the line-level outputs from the sub will use a high-pass crossover of 120Hz, whereas using the speaker-level outputs will adjust that high-pass filter based on where you set the crossover dial.
If you want to do #2 above, then you can either:
(*) Hook up one of the left/right outputs from the pre-amp to one of your power amps, then hook up the speakers that you want to play full range to that power amp
(*) Hook up the other set of left/right outputs from the pre-amp to your sub, then run the left/right outputs from your sub to the other power amp, then hook up the speakers that you want to play only the higher frequencies. This will only play frequencies above 120Hz on this set of speakers (if I'm understanding the manual), and it will play frequencies on the sub below whatever you set on the crossover dial (which I'd suggest be set to 120Hz).
-OR-
(*) Hook up one of the left/right outputs from the pre-amp to one of your power amps, then hook up the speakers that you want to play full range to that power amp
(*) Hook up the other set of left/right outputs from the pre-amp to the other power amp, run the speaker wires from that power amp to your sub's speaker-level inputs, connect your sub's speaker-level outputs to your second set of speakers, and adjust the crossover on the sub to set which frequencies are played by the sub and the speakers.
If you want to do #3, you can use either the line-level inputs/outputs on the sub or the speaker-level inputs/outputs on the sub. You can:
(*) If using the line-level connections (with a 120Hz crossover), then you would connect the left/right outputs on the pre-amp to the line-level inputs on the sub. Then, using two RCA splitters, connect the line-level outputs on the sub to your two power amps and connect the speakers to the amps as usual.
-OR-
(*) If using the speaker level connections (with a variable crossover), then you would connect the Rotel pre-amp to both power amps, connect both power amps' speaker connections to the Velodyne sub, then connect all four speakers to the speaker outputs on the sub.
