Help with choosing sub-woofers please

Whilst picking up the really helpful comments from all who responded, I have also been continuing my research on the net.  I think I have made some progress and am ready to do my first set up and listening trials if I can borrow the kit.

@jtcf     Thanks for your post earlier.  I am not sure you're right the KEF KC62 won't deliver the goods in my room.  There's a lot of new tech in there.  I've read four pretty positive reviews.  The fullest is at whathifi.com/reviews/kef-kc62 in Australian Hi-Fi, an offshoot of the UK mag What Hi-Fi.  This describes the technical innovations in some detail, which include two co-axial drivers and a cast aluminium box said to be very rigid - easier to make it rigid at this 10 inch scale. 

As to output it is true the KC62 is limited to 115dB and won't often be able to deliver all that.  But I will be listening to music only at reasonable levels, not cinema, so I certainly don't need gut-sickening boom.  There is 1000w of amplification to power its two small drivers; that should be plenty of volume for what I need.

One thing that alarmed me at first with the KC62 is that KEF claim it goes down to 11Hz.  This didn't seem possible for this very small size.  Nor do KEF specify how far down the output is there.  But the Australian reviewer did a lot of testing and publishes five graphs.  In the main one the frequency response is plotted for each of its 10 roll-off rates.  For rates between 40-60Hz - I am likely to start with 60 or 70 - the output at 15Hz is down only 8-9dB.  At 20Hz it is only 2.5dB down.  Even compared to much larger subs this is superb.

The KC62 has two co-axially driven drivers, mounted in opposition.  Two drivers firing 180º from each other not only damp vibration and rock and roll.  It also seems to me they will be capable of better room mode cancellation than a single driver.  I am going to start by trying two KC62s.  As well as experimenting with location, turning the orientation of each in each position should offer a lot more variability to experiment with.  If two work well over extending listening, then I may try adding one or two more.  Whilst not dirt cheap (R&D has to be paid for) these are priced at only about 40% of the Martin Logan Balanced Force, where I wouldn't really want to pay for more than two.

At just 10 inches cubed, probably in white not black, even four are not going to get in the way or degrade room aesthetics too much.

I will be reporting back and sharing.

One thing that alarmed me at first with the KC62 is that KEF claim it goes down to 11Hz. This didn’t seem possible for this very small size....At 20Hz it is only 2.5dB down.

Uh, yeah, I’d check that. If the company doesn’t specify a -3dB lower level, well, there ya go. I’d just say you can get a pair of SVS SB1000s that will get you down to an honest 20Hz for under $1000 for the pair. And, they offer a generous and truly risk-free, in-home trial including shipping both ways . And, they also offer an integration software app — not sure if KEF offers that, but that’s huge. Lastly, SVS is a business based primarily on subwoofers — KEF, not so much. Best of luck.

Thanks @soix 

The independent test by Australian HiFi covers this.  It may be a a fair bit down at 11Hz (in fact -22dB on that graph), but I will certainly take 8dB down at 15Hz and 2.5dB at 20Hz.  As good as anything out there and better than nearly all.  No-one else quotes figures at 15dB.  Not that there's much to listen to down there apart from rumble on your old Thorens and Garrards, ho ho.

By the way, the max output is 105dB not 115 as I typoed  Sorry.  But 105 is plenty loud.  One reviewer says that is eviction level.  Maybe.

You are correct, KEF doesn't currently offer digital set-up.  Looks like I'll be crawling around a while.

I am in UK so don't get US deals on SVS.  Some reviews report that the SQ is poor on SVS.

Yeah, I'll let you all know in a few weeks.

[clear thinker]   You and others advocate the crawl test as if all standing waves are bouncing around along the ground.  Surely they are present throughout the room, in which case we should also test whilst standing?  Would save my knees as well.

Your absolutely correct. Crawl Test is a misleading term / title used to search the net for directions and examples. My subwoofer manufacturer suggests the test near the beginning of the setup procedure. 

In my limited experience I usually elevate one subwoofer onto a sturdy table or side table at the listening position. Using the sofa or chair should be fine. While playing my subwoofers included Sweep Tone CD simultaneously through the main speakers and the subwoofer I walk the room listening and making note of the rooms nulls and modes. I measure the modes slightly varied levels using a Radio Shack SPL meter. I found actually crawling the room will yield slightly varying intensities and locations that I consider overkill.

As a result my two twelve inch subwoofers are asymmetrically located in the rooms two loudest modes. They're mounted on MDF platforms with 2 inches of high density foam using extra soft rubber casters rolling on a suspended wood floor.

Since every aspect of every room and system is unique, like speaker placement the Crawl Test is simply a starting point that I've found indispensable for any new installation. Best of luck with your project.