I need a loudness switch

Hi Scott,
Correct, you need loudness compensation. Our hearing is less sensitive to bass and treble at low volumes.  https://en.wikipedia.org/wiki/Equal-loudness_contour. Older preamps had a "loudness" switch but it was only really accurate at one volume. Tact Audio made a unit with "Dynamic Loudness Compensation." It changed the compensation curve dependent on the volume and it worked great except it was very expensive and the company is out of business. The bargain basement way of doing it is with  treble and bass tone controls. But, Audiophiles shunned these so most units do not have them anymore. What you are stuck with is the fact that every recording has it's correct volume. This depends on how it was mastered. You have to listen at the volume the music sounds correct until someone comes up with a digital processor that reintroduces Dynamic Loudness Compensation. 

Mike in NC. It is not just the bass but the treble also in almost equal amounts. 

Yes, the Loki would work. You would have to adjust it for every different volume so it would be cumbersome. You could also not construct accurate opposing filters to your hearing ( Fletcher- Munson Curves.) Not enough bands. The best solution is a continuous band digital equalizer programmed to follow the Fletcher Munson corrections with volume change. To my knowledge there is no equipment designed to do this at this time. The Tact 2.2X and the last TCS are the only units I know of that had this capability. Anthem and Trinnov should get on the ball. 20 years later and nobody has yet to match Radomir Bozevic's technology. Dynamic Loudness Compensation is a gas. No matter what volume you choose the music sounds exactly the same. It is all done in the digital domain at 192/48. There is no distortion, none. Analog is wonderful as long as you keep to the "strait wire with gain" principle. Every time you do something to the signal you add distortion. Not so in digital. You are just juggling numbers.

Mike, if you look at the Fletcher Munson curves the boost in treble is pretty high up and is less noticeable than the bass boost which is in a part of the range that is very obvious. So you are right in that the obvious part of loudness correction is in the bass but if you could AB curves you would note that the curve with the treble boosted correctly sounded more natural. 

As Scott has noticed it is not easy to do this in the analog realm. It can only be right at one volume and analog filters can play havoc with things like imaging. For decades the best preamps avoided tone controls and loudness switches. None of us would give an equalizer a first look never mind a second one. But with digital this has changed in a big way. As long as the sampling rate and bit depth are up there the benefits far out weigh the detriments. Analog guys have a tendency to be digiphobes. 
I think this is unfortunate. 

Tim, I'm all for the subs but since when is Class D the harbinger of good bass? Or good anything for that matter?