SPL Stereo Vitalizer Mk2-T (tube) a beast of a loudness control at $1295

Just get a Schiit Loki Max it has a Remote Control

https://www.schiit.com/products/loki-max

A link to a Sound On Sound review.

The SPL Vitalizer is a piece of pro audio equipment that at heart is an dynamic equalizer.  Describing it makes it more complicated than it really is.  In fact, it's fairly simple to use and it can definitely make most rock/pop type music sound better.  As a practical matter the unit needs to be positioned at the listening position in order to make quick and repeatable adjustments.  Not a problem in a recording studio, but could be in a home setup.

If you're an electric guitar player you are probably familiar with using the guitar volume and amplifier gain controls interacting for compression and overdrive effects.  The Vitalizer can do that (and a little more) with a more precision.

You may like what it does, but nothing in your description reminds me of a classical loudness control. As you may know, a loudness control is supposed to compensate for the Fletcher Munson curve imposed by our own hearing apparatus, where mid range frequencies are heard best and extreme low bass and upper treble frequencies are both rolled off. So a loudness control lifts both ends of the spectrum in relation to the ambient sound pressure level of the listening experience. That’s all it does. What you’re talking about seems very different, deliberately adding coloration that you find to be pleasant. Which is fine.

Just another audio band aid. A quick fix for a poorly constructed system. Focus on the synergy of the individual components, speaker placement and room treatment and you’ll have no need for such devices.

I imagine feeding this a Dolby B or C output from an old cassette player and being able to expand its dynamics.

DBX did that around 50 years ago. It has its limits.