@kalali Your situation sounds very similar to mine. I run two sealed 10” passive subwoofers and have experimented with a variety of setups to get the signal and send it to the subwoofers’ amplifier(s).
In theory, high level inputs have the potential for good sound but my amplifiers don’t have that feature and I’m not sure I would use it even if I had the option.
Through experience, the best way I have found to do this is to grab the signal off your main speaker amplifier binding posts, after it has passed through the amplifier. Drop the voltage to line level and run that through a dedicated subwoofer preamplifier. Dropping the voltage can be done via a voltage divider, with two carefully selected resistor values, connected to some female RCA connectors. You can build or buy this. It’s just a high to low converter.
There a several advantages to doing it this way:
1) It’s very simple. You need a high to low converter (cheap) and a second preamp (can be cheap or expensive or free if you have a spare)
2) You don’t mess with your main preamplifier impedances or muck up the signal at a critical stage in the chain. Your preamp is happy driving your speaker amplifier. Don’t add more chores to its list or you run a risk of degrading the signal. If you split the signal after your main preamp, you also miss out on point #3
3) Since you run the signal through your main speaker amplifier, you apply its sonic signature to your subwoofers. Both the mains and the subs have all the sonic signatures of the chain between the source and the speaker binding posts. This is very helpful, possibly critical, in getting the subs to blend with the speakers in the best possible way.
4) This setup lets you easily tube roll (or “preamp roll”) to tweak the bass to your liking.
5) Bonus: You can easily add a DSP module just before the subwoofer preamp. I have found DSP to be very helpful with subwoofer integration.
If you run more than 2 subs, it is “safer” to use splitters at this stage because you aren’t influencing the main speakers in the chance there are impedence issues and you aren’t interfering with your main preamp’s primary goal: driving the speaker amplifier. DSP is another way to get more outputs. Mine has 2 inputs and 4 outputs.
I have used a tube preamp to do this and it sounded really good. I have also used solid state preamps which sounded good as well. I am on the fence as to which is better for a subwoofer preamplifier. Very soon, I am going to use an iFi iTube2 preamp for this application. I will try to remember to let you know how it sounds. Please feel free to reach out to me if you think I can be of any help.
Happy Listening!
