I recently stumbled upon a video by John Hunter of REL setting up a REL 6-pack (3 subs stacked for each stereo speaker) which I found intriguing. My main system uses 2 BK Gemini II subs in stereo - one next to each speaker and passed at 48hz. I’m very pleased with the results (bass depth, stereo width, sound-staging) but, being critical, I can tell that the low frequency presence comes from towards the floor.
Any way, as luck would have it I made 2 x upward firing passive bass modules (I hesitate to call them subwoofers) over the summer for my office but this afternoon I had a chance to place each one, firing upward, on top of each BK. I used splitter cables for each channel to send a low level (pre-amp signal) to each BK and a Nobsound G1 100 watt (4ohm) monobloc to feed each bass module. Each Nobsound can be run full range or with LPF engaged, a variable crossover (40-300hz), adjustable gain and volume control. I spent about half an hour of tweaking levels - each Nobsound set to about 1/3 rd volume, 26 dB gain, and about 80-ish LPF - the crossover scale does not give numbers - my only criticism of the amps so far.
Net results are better than I expected and I can see that John Hunter is on to something with this, albeit at a cost! First, with the filter set higher than the floor mounted BKs, the bass modules help add punch to the sound. However the benefits are more than this. Soundstage width is greater but so is stage height - bizarrely this affects more than just the bass. Music has more presence and is more 3 dimensional but also has more tonal richness. I am also hearing new things on quite a few tracks and spatial cues seem more accurate too - the funny thing is that these seem to be at frequencies above where the subs and bass modules operate.
Now I’m not saying that my ‘home brew’ bass system will rival £50ks worth of REL stacks, rather from my quick experiment I can see the benefit of the REL approach - adding more bass in the vertical dimension but also having individual control of all crossover and gain setting for each subwoofer so that they can do different ‘tasks’ in the stack.
I would be interested to hear if someone else has looked into this and/or heard a REL stack in action.