If a pair of stereo full range speakers exist; so why not a pair of stereo monitors supplemented by a pair of stereo subwoofers.
{Minus 0.5dB @ 18Hz and with 116dB SPL}
Some experts mentioned that a summed mono bass output from the left and right channel and directed to a pair of subwoofers is just fine as most music recordings don't contain stereo low frequencies, simply a sum up.
I would love to be re-educated again by the most prestigious industry's leaders, on this subject.
But two subs for sure; better than just one. ...Even for just a two-channel stereo sound system. ...My own proven opinion.
* It would also be interesting to determine exactly which low bass frequencies (x-over) are sent to them dual subs.
Perhaps there would be a difference between say 80Hz and 40Hz. ...Or lower, 30-35Hz. ...That would have to be determined by the best match possible between the monitors (or speakers) used and them matching two subs.
Also, how do you connect them is important. Line level (main in/pre out - Low level unbalanced RCA or balanced XLR), speaker level (High level), sub out level at the preamp end, ...?
And, any equalizer involved?
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Generally a sealed sub is tighter, sharper, more natural in its descent towards the lows. And a 6dB per octave slope is a more gentle one for a better bland (natural). The right crossover is also very important, to the precise Hz point, like 36.5Hz x-over for example.
And nothing bigger than 12" drivers; a multiple of 8" drivers is good. ...Say a sub with three 8" drivers, or one with dual 10" drivers.
...For music listening that is.
<<>> I learned some from
Richard Hardesty, Mr. Boom. Ultra High-End audio writer and professional subwoofer/amp/preamp/DAC expert.
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http://www.audioperfectionist.com