What is the most dramatic way of increasing a speaker's Bass and Low mid?

If you place them in corners, assuming that works well, you might also spot-treat the walls to either side of the tweeter with thick foam. The idea is to selectively target the very wide-angle off-axis energy from the tweeter down at the bottom end of its range, where its radiation pattern is the widest. 

I have not tried this specific application of spot-treatment with foam because I've never been in the same situation as you, but I have used it in other situations.  Don't overdo it with the foam or you'll suck the life out of the sound.  I'd use a strip about 1 foot tall and maybe 6" wide, centered at tweeter height, right were any energy traveling sideways across the baffle would hit each of the walls that meet in that corner.  And it doesn't have to be foam - thick textiles work fine too. 

Duke

Dumbeat, in my experience cabling can make an audible difference which does not show up in the kind if measurements you are asking for.

Since I'm a speaker guy, I tend to give a lot of credit and/or blame to the speakers.  I do not look to cabling to fix my speakers, nor do I blame the cabling if my speakers suck. 

At a recent audio show we exhibited speakers that have a secondary array of drivers, firing to the rear, which are not very loud relative to the main speakers.  Their calculated contribution to the summed SPL is only about two-tenths of a decibel.  This secondary array of drivers was connected to the amplifier by a separate speaker cable from the main array.  This last detail is important to what I'm going to describe.

During set-up at first we hooked everything up with cabling that we had on hand.  Then several hours later the cable company we were sharing the room with arrived (Clarity Cables).  We changed cables one-by-one, because we had time, and because we wanted to listen for changes and decide whether the original cabling or the cable company's cabling sounded the best.  No one from the cable company was present for this.

I'll only describe what happened when we changed the speaker cables going to that array of rear-firing drivers, the ones contributing only .2 dB.

When we made that change, a harshness that I had been blaming on my speakers disappeared.  I had been planning to make a crossover change before the show opened the next day, and with the new speaker cables going to that rear-firing array, it was clear that no crossover change was needed.  I was amazed and relieved. 

The amplifier designer (Hans Looman of Resonessence) is the one who explained what was going on.  His explanation shifted my paradigm about cables.  He said that the original cables had been acting like antennas and picking up radio frequency signals, which the amplifier's feedback/error correction circuitry interpreted as distortion.  The error correction circuitry was therefore working like mad trying to correct this "distortion", and THAT was the source of the harshness I had been hearing.  It was coming from the amps, as they tried to correct for something their error-correction circuitry interpreted as a distortion.  The Clarity Cables did not behave as antennas and so the amplifier was no longer trying to correct for the radio-frequency "distortion".

Given that the secondary array of drivers was only contributing .2 dB, it would be unlikely that any minor changes in their frequency response due to the cabling change would be audible.  However if that cabling change made a significant difference in the amplifier's behavior, that difference would also show up in the main speakers, where it could theoretically be audible (and in this case, it was). 

I think this is an example of a non-obvious mechanism by which cabling can make a difference.  And no I don't have data to prove this, so you'll have to decide whether what I've described passes your "reasonableness" test.  Truth precedes observation, and observation precedes data. 

Duke

This past Rocky Mountain Audio Fest, our room was right next to the Schiit room. I was hoping to make a trade, a pair of my speakers for one of their amps. Of course the pivotal question was, are my speakers worth a Schiit??

Seriously, I think going for EQ is an excellent idea. 

Duke

There are a lot of tradeoffs in designing a woofer system and some of them may be in play in the differences you hear between the Merlin and the ATC.

For example, the Merlin is 89 dB efficient and the ATC is 85 dB efficient. The higher efficiency of the Merlin comes mainly from its woofer having a more powerful magnet. This more powerful magnet actually tends to reduce the level of the bass (and even lower midrange) region relative to the rest of the spectrum. If the Merlin’s woofer is then placed in a cabinet that is tuned fairly low, there will likely be a "sag" in the response curve above that, and this "sag" can extend up 3 octaves or more above the port tuning frequency.

If this is indeed what’s going on (or at least part of it), imo it’s the sort of thing that EQ can help with.

Duke

In my opinion EQ is much more likely to help than is bi-amping. In your situation you need to be able to make finer adjustments than simply changing the relative levels of woofer and tweeter. That would help, but I think you’d still have weak upper bass/lower mids relative to the top-end of the woofer’s response.

Regarding SET or OTL amps, I looked at the frequency response and impedance curves in Stereophile’s review of the VSM. Relative to a solid state amp, a high-output-impedance (low-damping-factor) tube amp will put out more power into an impedance peak, and less power into an impedance dip.

So in this case the result would be more output south of 100 Hz; a bit less output between 100 Hz and 800 Hz; and a bit more output north of 800 Hz. Imo ONLY the increased output below 100 Hz is likely to be beneficial. North of 100 Hz, I think the net effect on frequency response would be counter-productive.

While I am a big fan of these kinds of specialty amps (I’m an Atma-Sphere dealer and design my speakers specifically to work well with them), I don’t think this is the solution in your situation. I’ve heard Merlins driven by a low-damping-factor tube amp and to my ears the combination still had the characteristics you have described.

Regarding adding a bucking magnet, I assume that "raises the output by several dB" comes from increasing the motor strength. I do not think that is the solution either, for reasons described in my previous post.

I think EQ is probably your best bet for getting the tonal balance you want from those speakers. 

Duke

My suggestion would be to first see where you end up with the Schiit Loki in the signal chain. If it solves the tonal balance issue with your current amp, then it will also solve that issue with an SET or OTL amp (unless the knobs are already max’d out). And from there, you can probably guess which 30-watt amp I’d suggest.

Duke

I'm friends with an experienced technician (who does warranty service for several brands) and he is very complimentary of the parts quality in VAC amps.  

Duke