Simple Sub-Woofer Driver

I've only ever used active subwoofers and they have all had their own crossover. So I would suggest full range output. I'd also suggest stereo output instead of mono for added flexibility as most subwoofers have a stereo input and do their own signal summing. It also gives the option of 2 subs (i.e. left and right).
I'm also on the lookout for a new subwoofer, my BK XLS300 is completely out-classed by a pair of Proprius and PMC speakers

1. Yes, most if not all powered subwoofers do have an inbuilt crossover. I believe these are most commonly Linkwitz-Riley - some research may be needed to check.

2. I would recommend a high-pass filter because in my experience, removing the lowest frequencies from the main speakers can be highly beneficial - less work for the amps to do and smaller cone movements for the mid bass drivers giving a tighter, cleaner sound. Without a high-pass filter, lower frequencies from the main speakers are likely to add to those of the subs in a more unpredictable way.

There is no definitive answer as to the slope but I have found 4th order 24dB/octave to give a good blend and I believe this is now fairly common with powered subs themselves.

3. Variable crossover frequencies from, say 50-120hz if possible with the range 80-100hz being ‘typical’ & the lower the better, depending on the capabilities of the main speakers.

Before I moved to an active crossover system, for many years I used a small Miller & Kreisel passive high-pass device between pre-amp and speakers. This contained a 2nd order passive crossover set at 80hz and controlled frequencies/crossover for the mains/subs. It did a very good job and avoided many of the frequency issues found around the crossover point. However it did not fully address phase and did nothing for timing which is why I moved to an active system.

This device went out of manufacture many years ago but the manual can be found here.

4. If possible, I recommend the facility to manage a pair of subs - either stereo or summed mono positioned differently in the room can have a very positive impact in taming room nodes.

Richardl60 wrote: My perception is that Subs would be power hungry, is that correct or a false impression.  Depending on the answer would the amp need more grunt than the Proprius, albeit in a limited frequency range (sorry have no knowledge of impedeance of subs)?   Richard

I wouldn't think it would require more power than the left/right channels, but then again there will always be bass-heads, as you commented.

By making it a "pre-out" level, users can choose their own amp or active sub accordingly.