Just curious if the answer would change if I were to upgrade the B&K 200.5 to say a Parasound A52+
The B&K's "signature tone" is said to be warm whether it is due to harmonics profile or due to rolling off the highs. Combine that with the Marantz which is something people also perceive to be warmer for reasons that is difficult to correlate with current set of measurements. You may also have a "deadening room" acoustically. Combine that with a reference target curve that slopes down and you have a set up that tends to lean towards warm or at least attenuated highs. Not having sufficient highs (within the audible range of one's ears) can be perceived as muddy or not having sufficient detail. While some people may prefer this for music, it won't be satisfying for HT use. A better perception as you described by flattening the target curve suggests that this may be the case.
A REW/Audyssey measurement of your entire chain would show what you are actually getting out of the speakers if you are able to do that. Just the mains should be sufficient. The reason for this is to figure out if your need for "better mids and highs" is based on a deficiency in your equipment or from your auditory, room mode perception that might even require more pronounced mids and highs than flat. The solution would depend on which one is the case.
If the measurement out of the speakers is relatively flat and yet you perceive it as muted in the highs, then you really need equipment (or EQ) that is "bright". More neutral equipment is not going to help you. But getting "bright" equipment may have come with a side-effect of inducing fatigue or sounding harsh.
So, simply buying equipment, however well they may be measuring, hoping it will make you happier is not very efficient. But with the tools at our disposal, we can do the equivalent of customer tailoring than buy off the rack hoping it fits better.
What I would suggest is doing some experiments with Audyssey in the Marantz and using the full Audyssey app to change the target curve in different ways including removing any notch filter in the mids-highs it may have put in and perhaps even boosting the mid-highs above flat. Think of this as a fitting trial in tailoring. You are not trying to get it to sound perfect but an understanding of roughly what is needed to approach a sound that you prefer in your context of the whole chain and room characteristics. This is primarily about tonal balance not noise or distortion. Nothing you have or buy will be affected by the latter metrics.
Once you get an approximation of the tonal balance that "sounds right" to you, then based on that "fitting measurements", you can figure out what equipment change if any is needed and select equipment accordingly.
Just my suggestion.