the NAD drive the Monitor audio B4 loudspeaker through NAD 541CD and bluesound Node 2i on low and mid volume the sound is close to my opinion
I think you need to go and listen to a few options,
The Monitor audio B4 are good but i found them very wolly and a little congested, i wouldn't say the NAD is closed in a low volume at all far from it and is far from your weak link.
I think you're better off looking at some new speakers here IF you wish to change something. This is where you're going to notice real improvements/flavour changes the most. Amps, even today's offerings don't bring massive sound improvements if at all. Certainly not night and day, i think you need to temper your expectations here a little. Though it could be you the little NAD needs servicing, what is it now 21-22 years old? so it might be more economical to replace.
If you must buy another amp another option is to sell both the NAD and bluesound node 2i and replace with a power node 2i cutting your box count in half and you may actually have money left in your pocket or costing you nothing at all. The NAD 320bee can fetch tidy sums on ebay as can the bluesounds. Just a thought!
do try and listen to some options though if you can.