BrianM -
I'll try to answer, but the topic is a little more complicated than a simple answer might do your questions justice.
I understand that the QW's extended bass could/would make a sub redundant...
No necessarily. The TL cabinet in the QW's allow the woofer to play lower (by exciting air in the TL cavity) than it would in a normal ported cabinet. The driver starts falling off at basically the same frequency as in a ported cabinet, but it falls off much more gradually. The effective bottom end is highly dependent on room placements, room modes, etc. So you end up with usable bass in the 30's, but it may be down quite a bit. If you want flat response in the 30's, a sub would probably be required.
But the QW's will have a VERY gradual fall-off of bass response while the sub will have a rather sharp slope on the top end in comparison. So, without some sort of signal processing (via TacT, DEQX, Behringer, or similar), it will be difficult to get the two to mate with resulting flat response (not to mention phase relationships).
but, and I'm sure this is a dumb question, why does it always seem to be 50Hz that's the magic cutoff frequency? If QWs play down to 35Hz why can't a sub cut out right there instead? (It's not like I've done zero reading on this stuff, but I hadn't come across that particular answer yet.)
If you want flat response, you need to have the crossover to the sub set high enough so that it fills the area where the mains start falling off. 50Hz is roughly the area where the driver starts falling off.
Here is a FR plot I simulated based on Dave Ellis' 1801 cabinet. So this is roughly how your 1801 W18 response will look:
As you notice, the F3 is just under 40Hz, but the falloff begins at about 50Hz. Keep in mind that at the -3db point, the output is half of the output at 0db. As you can see, the response starts to fall off around 50Hz (your "magic cut-off frequency').
With the QW's, the fall-off begins at roughly the same place, but falls off MUCH more gradually. Since the top end of a normal subwoofer plate amp fall off much more steeply, it would be more difficult to smoothly integrate the two slopes.
I hope this makes sense.
Jim's building me some 1801s, so this is a topic I'm trying to bone up on.
By the way, I shot sealer on your cabinets yesterday and they are turning out stunningly. I'm still waiting for Dave to send the kit, but we're making great progress!
And I have one other question: what does speaker sensitivity have to do with the type of music you listen to? HT2s are better for classic rock because they're more sensitive?
Sorry I confused you. Sensitivity has nothing to do with the type of music you listen to. I just thought I'd mention it because it is one advantage of the HT2 design. As for why they would be better for classic rock, that is a matter of personal preference. They seem to have just a little more mid-bass presence that some people would prefer to hear with rock music. This is just a matter of personal taste and is why I made the comment.
Why does 2 mids make a speaker more sensitive anyway?
You basically have two woofers pushing twice the air. In theory, you should gain 6db with an MTM design over an MT design.
What is the sound difference between the HT2 and the QW? Just bass? Ok that was more than 1 question...
That is an extremely difficult question to answer. It is not just bass. An MTM design sounds slightly different than a TL design. Every listener will have a different interpretation. My personal observation is that the QW will sound very smooth and balanced. This is what I would prefer for classical or jazz. The MTM design seems to have a little more mid-bass punch (for lack of a better term) that I would prefer if listening to rock music. But we are splitting hairs here. Either speaker would work very well with any type of music. The original question related to my recommendation (which may be different than Dennis Murphy's or anyone else's) and I gave it based on my personal preferences.
I hope this all makes sense.
- Jim