I am planning on building an accoutically treated room mainly for music listening. I have lots of unused space in the attic therefore thinking of installing the manifold there. The ceiling is made of chipboard insulated with 30cm thick glass wooll.
Currently I am using the Audio Physic Spark 3 speakers and always felt they were lacking the bottom end. Having in mind that the room will be rather small (350x500x280cm), the choice of infinite baffle subwoofer seems to be the only affordable option for me.
As I am planning to replace my current stereo amplifier I am not mentioning it.
Now the question regarding the equipment needed for the setup. If I chose the 2x18" manifold, what would be the best way to connect the subwoofer amp with the main stereo amp to match their SPL? What kind of wiring connectors should both the amps have so I could manage the volume using single remote control?
Typically people are using the subout from a receiver or Prepro to feed the signal to the sub. Those have separate output control levels for each different speaker set, ie, L>C>R, rear effects, and sub. This allows the relative output between speakers sets and well as all volume levels to be controlled with the main remote.
Does your stereo amp have pre-amp outputs? If so those can feed an external crossover.
If one understands electronic circuitry and the stereo amp isn't all large scale integrated circuits sometimes an engineer can go in and add a line level output to an existing amp. Given the hourly cost of servicing electronic components this option isn't usually practical
Another option is to use what's called a DI Box, (direct inject device). These were developed for prosound use. The speaker output from a power amp is wired to the DI Box. The box changes the AC output from the power amp to a line level DC signal that can them be sent to a crossover. An issue with this option is that all this manipulation of the signal doesn't make for particularity high quality sound
So, sorry no, in most instances what you're wanting to do isn't easy or inexpensive
You could use a MiniDSP 2x4 HD between the preamp and amps. That would give a total flexibility in merrying the stereo speakers and subs [time alignment, crossover (freq and steepness), parametric EQ, etc). You could even do stereo subs if you want.