Best balanced linestage/preamp for $4000 or less ???

Atmasphere - do manuals generally indicate if the equipment meets the AES 48 standard?

@oem-wheels 
Probably not. I would contact the manufacturer. It they can't simply say 'yes', the correct answer is likely 'no'.

um , maybe not ... $5600 with phono... don't need that part

The MP-3 is also available as a line stage.

@oem-wheels If you plan to get an MP-3, it has no worries driving a 10K load or even a lot less. IOW it can drive the sub and your regular amp no worries. The biggest issue is whether the sub has balanced inputs- many don't.

A useful thing to check for when looking into balanced preamps is whether they support the balanced standard (also known as Audio Engineering Society file 48).

The reason to support this standard is to eliminate the sonic artifacts of the interconnect cables that will otherwise occur. If you've ever had to audition cables and maybe paid extra to get the one that sounded right in your system you know what I'm talking about. Imagine a system where all cables sounded as good as the best you've heard regardless of price; that is what the balanced standard is all about.

Here is the standard, in a nutshell:

Pin 1 is ground, pin 2 is non-inverting (in the US) and pin 3 of the XLR is inverting (in the US; in Europe these two are reversed).

Ground is ignored- no signal return currents are conducted in the shield; the shield is for shielding only! This is the area where most high end audio preamps have problems and is why you hear so much variance in conversations about whether balanced is better or not.

The output of the balanced source is low impedance. To this end, it should be able to drive 2000 ohms without loss of bass or increased distortion. The lowest input impedance of solid state amps tends to be 10,000 ohms and 100,000 ohms is common with many tube amps so you might think this unnecessary . But this low output impedance is important if cable immunity is to be maintained.

I was going to suggest a used Atma-Sphere MP-3, as Charles did, except that I’m pretty certain it only provides a single pair of XLR outputs, and no RCA outputs, at least in its standard configuration. If you can find one that you would want to consider, though, a call or email to Ralph Karsten at Atma-Sphere would be in order, to see if he could modify it to meet your requirements.

@almarg , @oem-wheels , the Atma-Sphere MP-3 is stock with dual outputs. Older units might have single outputs (dual outputs were optional), but extra outputs can be added inexpensively.