Weak link

Hi all, I am relatively new to this world, and I have a question which, accordingly, might seem easy to answer to some. At least I hope.

My set-up is the following:
Onkyo HT-RC360
Carver A500x
Mirage M1
cables : from Carver to M1 : blue jean AWG10, biwired.

Onkyo -> Carver : this is a bit unorthodox, but I use a speaker to line level converter and generic cables to connect the converter to the carver.

While I really like the sound coming from this system, which is an improvement over having the Onkyo driving the M1 directly, I am found wanting in terms of high-frequency details coming from the speakers.

I read that the carvers deliver in general in terms of power, but not in terms of details, could it be the main culprit?

The other route is to get a preamplifier (any recommendations for this setup?) I am sure there are many weak links in this setup, but I am not quite sure which upgrade would yield the most audible improvement.

Thanks to all!