Mono block Preamp

Lamm LL1 Signature is a true mono block preamp. completely separate left and right dual chassis. no remote volume control.

https://positive-feedback.com/reviews/hardware-reviews/lamm-ll1-1-signature-dual-mono-preamplifier/

Gryphon Commander likely comes next closest, as it even has two power cords....and a separate power supply connection for the control processes. 

https://gryphon-audio.dk/shop/preamplifiers/commander

when you consider the switching and gain functions there is a need for a common control chassis. the Lamm is a bit too ’hair shirt’ for many with no remote volume control. a non-starter for most people.

YBA signature preamp. As i can remember in 6 chassis, even had separate power supplies for phono stage.

Coincident Statement linestage is true dual mono, to get all those parts into one chassis would require it to be quite large and affect performance, what with emi/rfi. Two chassis, one control unit, signal path only, other for power supply. I think this better configuration vs power supplies in both chassis, just for the emi/rfi alone.  The two mono volume selectors can be a pain, and no remote available.

There are several mono phono stages.

I believe Conrad Johnson offered a complete dual mono preamp which came in 4 cases.