looking for upgrade suggestions-- more soundstage

I have a NAD 352 integrated amp driving Tetra 120u monitors (simple and fantastic!) in a small room. Bedrock cables and Hero interconnects. In 2008 i upgraded from a NAD 521BEE cd player to a CYRUS 8X with separate power supply--vast improvement. I recently moved from lead-shot weighted Sanus stands to four inch spiked maple slabs on the floor (a la Mapleshade)--subtle but definite improvement. i was very leased until...

I bought a NAD 747 home theatre reciever for a video set up in another room and made the mistake of testing it on my audio system--big improvement over the NAD 352! more holgraphic, better sound stage.

So, do i add a preamp to the 352 or replace the whole thing with a better integrated? I'd like to keep it between $700 and $1300. Prefer used equipment but new is ok too.

Thx in advance.
I think you should consider selling the 352 and moving up the ladder. Look at integrated amps from Creek, Exposure, Rega, Musical Fidelity etc. Pretty easy to find in you price range.