Where can I start to improve my sound the most 2ch

Room acoustics.

The CD player in your system is where I would start. It does not seem on par with your other components.

I started with a Cambridge Audio 300SE and thought it was great. Then I tried a California Audio Labs CL-15 and the 300SE seemed grainy, lacking dynamics and slow. I now have a Merdian 508.24 and the CAL has moved to system B.

I would also suggest speaker placement and room tweaks if you have not tried those yet. They can dramatically improve your system sound.

Concur Room Acoustics

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Realise that you have a surround system.

Then stop playing CD's in two channel, get the contour center and some Audience 52 rears and leave two channel behind.

Put this mantra into your head:

LP's= 2channel
CD's= Multi-channel

The Arcam is one of three or four receivers that can playback multichannel on an extremely high level. Proper set up will give you a wide deep soundstage with a sense of real life, no brightness or harshness and when you get used to all the info being around you, it will reveal much more detail in a subtler way. You should be able to encapsulate yourself in a 360degree soundfield with a very real and defined wide(or small)front stage that defines the space the recording was made. Any obvious surround issues are setup problems not equipment problems. So don't give up easily.

Try to get each speaker to image one to another.

Dynaudio makes for excellent surround system speakers, So your task is simplified by making a wise initial purchase.

Ze Ruuum! Ze akoustiks!

The source equipment, the cd player, is the place to start. A great player with a mediocre amp will sound significantly better than a mediocre player with a great amp.