To clarify my previous post just to make sure you are understanding correctly, on a 4 channel amp you would do
-channel 1 – Left speaker
-channel 2 – Right speaker
-channel 3/4 – Sub (either bridged with 1 driver/2 drivers wired together or with 2 drivers with 1 per channel)
you need each channel to be able to provide full rms power of the soundstage 15. Sub driver configuration can be decided later when it is designed based on what amp you were able to get
As im in high end pro audio the amps I know and would use myself are probably out of your budget range but ill show you any way so you get an idea of power and features
One option id use is the Lab Gruppen PLM 5k44. This is a DSP amplifier capable of 1250W per channel at 8ohms. This amp doesnt support bridging channels so I would do 2 drivers on 1 channel each for the sub
https://www.labgruppen.com/product.html?modelCode=P0CN9
Another option I would use is the Powersoft X4. This is also a DSP amplifier and is capable of 1600W per channel at 8ohms and upto 6000W bridged at 8ohms (these numbers may be on a 3phase power input so lower on standard power input but still plenty for your use)
https://www.powersoft.com/en/products/touring-amps/x-series/x4
Both of these amps are in the £5500-£6500 price range so almost certainly out of your price range and might be harder to find for someone like you as they are high end pro audio amps but they give you an idea of the kind of power and features you would be looking for. You are probably better getting 2x 2 channel non DSP amps like crown or behringer with the same sort of power/bridging capability (still connected the same way) and external dsp to do your crossover and EQ adjustments e.g. mniDSP(s)
Others on the forum will be better knowledged to assist with this type of amplifier choice @tvor-ceasar @ajc9988 @123toid any of you have anything to add to this discussion regarding amp selection?
