Although this may be of a different light, I did have the same experience with my Manley Stingray. Again, like your Manley preamp, the minimalist design of the Stingray seemed to drop off the treble too. In my case, I changed the resister leading to my speakers tweeters. Hence, the treble increased and the presentation was more balanced. I will say aside from that minor adjustment, the Stingray was a fabulous product in my system. Actually, without tone controls on preamps, I've found changing that resister to be a God send. My speakers are Tyler Acoustic 7Us (MTM design) and Ty, the owner of the company walked me through the the procedure over the phone as well as sending 3 pairs of resistors to suit my taste. This may be the last option you'd like to consider, but it worked well for me. Not only with my Manley products, but as mentioned, in any circumstance where whatever preamp I'm using at the time could use a twist.
Have a good day,