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Birds of a feather tend to flock together ... I wouldn't mix a vented sub with a sealed acoustic suspension type of sub and I wouldn't mix either of them with a Transmission line or Open baffle type sub
Why ??? ... it's simple .. they all roll off at very different rates per octaves with different angled slopes under their respective resonate points
A ported sub's slope rolls off at 24db per octave ... a sealed sub rolls off at 12 db per octave and a Transmission line sub rolls off at 6db per octave ... so if you had a same size sample of each of these type subs and a target frequency that was below all those type subs resonate points of say 30HZs ... the ported sub would have a slope that rolls off at 24db per octave .. the sealed slope would be down 12db and the transmission line sub would roll at 6db per octave
At that 30HZs the ported sub would be 12db down to the sealed version and 18 db down to the transmission line sub at the same 30HZs frequency ... they would all have different outputs below 30HZs due to their slopes and rate of roll off
Because their slopes are so different in rate .. it would be very difficult to set a cross over point to blend the two different types of subs and even more difficult to blend the subs with the main speakers as all their slopes would all be very different and rolling off at different rates
Also if my main speakers were ported I would use a ported sub and if your main speakers were sealed A/S type I would use a sealed sub and the same sing goes for the transmission line main speakers and subs
When your main speakers are the same type (ported .. sealed .. transmission) as your sub(s) it's much easier to blend the crossover point as the mains roll off at the same rate to the sub(s) rolling in as they (mains and subs) do so at the same rate because they have the same slopes under their resonate points ... 24db .. 12db .. 6db
Using two similar type subs as your mains will be very helpful in reducing peaks and nulls in your listening room when placed properly ... this set up will also reduce the amount of electronic intervention needed
Below 100HZs the bass frequency's lengths peak to peak become to long to trap effectively with 4 to 6 inches of conventional resistive bass trapings ... so you need to EQ most frequencies below 80 to 100Hzs ... much cleaner to use two subs properly placed as they will deal with both peaks and nulls passively ... where as with electronic intervention you should never boost a NULL as it requires to much power and can clip your amp
One more point I've experience in my rooms is 12 inch subs have excellent slam and impact (Mid Bass Attack) with great jump factor ... but 12 inch subs do not create or define the dimension's of the venue ... by that I mean they fail to create the acoustic space of the venue where you can actually sense the ceiling and walls
On the flip side of the coin ... 18 inch subs do a terrific job of defining the space and dimensions of the venue but have very little slam and impact ... no jump or startle factor
Lets not confuse LOUD with the speed of SLAM
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I recently bought a second REL that is of the same generation as my original Q150e. The newer one is a Q108 MKII that is a "down firing" 100 watt 8" instead of the 150's 150 watt front firing 10. I bought the 108 to stick in my window to help my deck purposed speakers (I stick those in the windows also…firing out onto my deck with their dedicated amp, and take them out of the windows when not in use…a thing that works). The 108 when not serving the deck now adds to my 150 and the combination is fabulous (both subs get the same signal from my main tube amp, as hey…I'm not an idiot!). I think the multiple sub thing tames some standing waves, adds to what already was a balanced sound, and makes me seem like I really care, although nobody else seems to care until I point these things out. Still, worth it.
Two subs can exacerbate what might be a bass heavy room, especially rooms with flat ceilings and parallel walls…you just never know. However, a second sub can sometimes have the effect of canceling or minimizing standing waves as it may be somewhat out of phase with the waves rolling around the room…this is what happened in my Essentially Sacred Listening Chamber of Wonderfulness. I think you simply have to move subs around some to see what's up, but I'm also still a proponent of using one sub as it's often plenty. Note my listening room has a slanting ceiling that is around 9 feet on one (the hifi rig is on the shorter wall) side and 15 feet or so on the back wall…that could be what saves me.