What reamp boxes are there on the market?

Holy... that Millenia unit looks almost perfect. It even has a power soak for direct amp recording. It's essentially one unit that has everything I've been buying over the last few months in-built. If only I'd known!
 
A thought occurs - couldn't you just solder e.g. a 24k resistor across a cable's jack plug? Plug that end into your amp and the load on the DAW's output would be 24k in parallel with Zin of the amp or pedal you're feeding. That would typically be 1Meg or 470k, so total Zin would be slightly more than the 22k minimum most line-outs are rated for.

Perhaps someone could spot a problem with this I haven't thought of?

Only question I can think of would be whether the device you're feeding cares what impedance it sees upstream. AFAIK tubes wouldn't care - after all the valve grid passes practically no current anyway, it only cares what voltage it sees. Same would go for FETs. BJTs might be different. But they suck anyway :p

Edit: No, actually scratch that idea - there would be local feedback from the cathode to the grid via the cathode resistor and the grid resistor (which sets the Zin). Shunting the grid resistor to a lower value would inrease the NFB resulting in a reduction of gain and less of those nice tube-y non-linearities. Bugger. Oh well...