Thinking I need a Sub...

Digital bass management compression/limiting nannys are not necessarily an advantage, they're simply designed as a "catch all" remedy for components that are badly matched to a room's acoustic properties. Of course things need to be "used properly," otherwise things are simply improper, if not tawdry.

Neo,

Since Bob and Wolf (both pretty convincing contributors here) are providing conflicting advice, I'd suggest that you find a good high end HT installer/retailer and do an A/B comparison with Digital Room Correction and without it. That's a simple bypass button and a very clean A/B.

You've got to find the right showroom so that the sub placement is optimized prior to room correction EQ. Then, you can decide for yourself.

For the record, I've done that in my dedicated listening room with two different pairs of subs (Rythmik and Velodyne) and in my HT room with B&W subs. The positioning of the subs in the listening room was optimized both by ear and - on a second pass - with the aid of a real time analyzer. In the theater room, the position of the subs was largely dictated by the room layout, so there was much less flexibility in optimizing location.

In all cases, room correction made a dramatic improvement - to my ear. In fact, by my reckoning, no other change to my system over the last two decades has come close. But there's a caveat.: Since Wolf's mileage seems to vary from that conclusion, be aware that yours may, too. Which is why an audition at a retailer who can do it right is probably your best bet.

Marty

As to the active/passive (distortion) debate, that's a tougher A/B since most systems (that I know of) kill the sub output entirely when you remove the active low-cut filter from the main speakers. That means you'll probably have to re-wire the system before you can compare active crossing to passive which makes the judgement more difficult IME.

I find the theoretical arguments for actively crossing a system pretty compelling, but my experience suggests that it's very much case by case dependent upon the main speakers. I've done it both ways and had good success with both - but the best results I've gotten have come from actively crossed systems. If in doubt, I'd say cross actively but would note again that YMMV.

I agree that when a room is overloaded with subwoofer bass energy resulting in crappy sound, the easiest solution to this issue is digital limiting. My point has always been that great sounding realistic and musical bass can be achieved with a single sub used properly, and since many think "more must be better" I understand why Taming Overkill With Digital Limiting/Compression has become popular. Every poster I've noticed advocating digital management has large, powerful, and often multiple subs that should utterly overwhelm the listening space without active limiting, which seems extreme, somewhat non musical, and unnatural...like buying a 600 hp V8 car and installing a device to disconnect 6 of the cylinders most of the time.

To clarify re: "overloading the room".

That is not - and never has been - the issue for me.

When setting up strictly by ear, I always start by attempting to level match at the x-over point. I have generally been good to within +/- 5db in that job (the most critical set-up element IMO as smooth response thru the "hand-off" is critical to seamless integration). That result is always verified by measurement after the fact (and subsequently tweaked).

The issue with the approach is generally "lumpiness" - I've always had 10+db peaks and/or nulls without EQ - despite exhaustive placement optimization efforts. Sometimes there's a somewhat excessive bass balance overall, sometimes it's a little light overall. It's never particularly smooth - as measured or (more importantly) as heard in A/B comparison with subsequently EQ'd bass.

The issue is virtually moot with bass management software. The Audyssey set-up protocol involves setting the average aggregated sub output at 75db per measurement on a bass sweep. The resulting measured on-axis FR is essentially flat across the entire range of the system. The difference between pre EQ and post EQ is not overall bass level, it's freedom of peaks and valleys in the response. The audible difference is not in overall bass level, it's in articulation through the bass and mids.

While my subs have the capability of producing much higher SPL (the 75db start point results in the gain pots being near the low end of their potential travel), the benefit here is less driver excursion and distortion. A peek at measured distortion tests will reveal VERY large values (often well north of 30%) for most subs as driver excursion (and SPL) increases, but I'll pass on the debate as to the audibility of this phenomenon. I will only note that the remaining potential clean output of the subs that is foregone by my set-up ensures that I've minimized distortion - for whatever you think that's worth.

The bottom line is: this ain't about overloading the listening room

Listen for yourself. Wolf is an experienced professional sound guy and I respect that credential, but I've always been surprised by his emphatic stated preference for non EQ'd subs. As noted, in my book this is a black and white issue, but - as also noted - I appreciate that other people's books are different from mine.

As a side note, I usually have a guitar in my hand 3 to 4 hours a day and feel pretty good about the subjective side of this evaluation as well as the measured evidence, The good news is, this is a very easy, very clean A/B testing opportunity - as long as there's a good, high-end AV place locally. IMHO, everyone who cares should satisfy themselves on this one.