I need a loudness switch

"My god man, get a Schiit Loki!!!"

I'm also looking for a similar solution as the OP and am wondering how would the 20Hz control on the Loki blend with speakers that only go down to 35-40Hz? My concern is getting a "single-note" bass unless they have implemented a nice slope which could help with the transition to higher frequencies. Any input from folks using the 20Hz control for boosting the bass would be helpful.  

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

Or you could try one of these, at no risk (cost).
https://www.musicdirect.com/accessories/Jolida-Foz-SS-X-Sound-Stage-Expander

I second the Jolida unit
I would not be without it now.
Love the fact it actually has three inputs so all of my analog front ends are plugged into it, cassette and reel tape which on vintage units have notoriously low outputs to begin with.
Now it does not boost treble as such, but the "wider image" dial can certainly give that impression.
Yes you have to adjust the settings at times for different sources or albums but you would likely have to do that with a Loki or any tone control piece.

Plenty of good reviews on the Black Ice and even a thread in this forum.

https://forum.audiogon.com/discussions/jolida-black-ice-audio-foz-ss-x-sound-enhancement?highlight=j...

     Yes, the old loudness controls boosted both the bass and treble to compensate for the Fletcher-Munson curve reality of human hearing.

    Strange, but this is also one of the 1st qualities I noticed with all the high quality class D amps I've owned.  The bass, midrange and treble balance seemed to be properly balanced to me no matter the volume.  YMMV?  Not sure.

Tim