High end sound

Commiserations 

Must be very frustrating but happily lots of good suggestions and advice, but what will work and what won't will only be apparent when you try them so the priority needs to be what should I prioritise in the interests of cost effectiveness.

Speaker positioning is a no cost option so no 1 in my book. No 2 I suggest is to clean all the connections with Deoxit or similar

Do you have a dedicated power supply to the system? If not and it is an option very cost effective IME.

The amp at 20 years old would almost certainly benefit from a service If it works fine. If not will improve its value for trade in or sale

Cables are never straightforward and can be costly. Finding the ones with the right synergy with the rest of the system needs a home trial but can be very successful.

I can endorse the Goldnote PH10 phono suggestion from personal experience. I had one and now have the PH1000 plus matching PSU.

Hope that helps. Take your time. Patience always pays.

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When was the last time you had a hearing test?  High frequencies are the first to go and can make you perceive your system as dull

Hmm yeah I agree it could be high frequency hearing loss because to be honest, even though I do agree Caps go on Amps, Bryston sound signature is on the sharp bright side of neutral. I do not believe its Speaker height because you have the Magnepans and if you have out 3 feet from the wall as you stated, positioning does not seem to be the issue for the dull sound you are experiencing. Cables could make a difference but cannot be responsible for your system sounding dull of a sudden. Now the combo of a Bryston solid state with a tube preamp I am familiar with and have swapped out the tubes to the point of tedium when I used the combo that was similar to yours. Great fun. It just so happens my Former BAT preamp utilized the very same tubes and I do know the tubes make a huge difference in the overall sound signature. Have you recently changed the tubes or have you been using the same tubes for a prolonged period of time? Have you gone up to the speaker and listened for any strange kind of Whiney sounds/creaking sounds (ie microphoney)? It would be interesting to hear what tube brand you are using and how many tubes the preamp requires.

Ditch the Maggies. Get a pair of Philharmonic BMR HT towers. Set them up properly and thank me later. The only speakers that compete with their distortion levels are big dollar Magicos, Rockports and the like. 

If you insist on staying with Maggies, perhaps give the .7 a try. I had both the 1.7i and the .7s. The latter were considerably more dynamic in the mid bass and had superior coherency. I could never get to where the individual drivers sounded as one whole with the 1.7is.