Help me spend 600

Hello All,

I want to get more out of my Rega Planar 3. I've got $600 at the most to spend. I'm not sure if I should first spend money on the cartridge, a phone preamplifier, or a better integrated amp (eventually everything will be upgraded). And I'd be grateful for any suggestions you have for each category. Here's my current system:

Rega Planar 3, RB300 tonearm, Express Machining Heavyweight and VTA adjuster, Shure V15VxMR (got it for a song from a friend who works at Shure; I also have a Rega Elys w/ about 200 hours on it)
Cambridge Audio A3i (built in MM stage, 50wpc)
PSB Image 4T (biwired w/ DIY cable)

I listen to roots blues (Fred McDowell, Son House, etc.) and a mixture of modern pop. My listening room is approx 12'x24'.

The Shure has an output of 3mV and a recommended load of 47k ohms in parallel w/ 250pF. The Cambridge phone stage has a input sensitivity of 1.75mV and an impedence of 47k ohms.

Due to space considerations, I'd like to stick w/ an integrated (tho' this is not a firm commitment), and I need an integrated (or pre amp) with at least five inputs (six would be better).

Possibilities
Cartridge (would prefer to purchase new)
Rega Exact
Phone Stage (used would be fine, and I'd like one that's switchable between MM and MC)
Black Cube
Monolithic PS-1
Rega EOS
Gold Aero DB45
Integrated (used is just fine, a switchable MM/MC phono stage would be a necessity)
There seems to be a lot available at this price, things like Carver, Arcam, Rega, etc.