Possible to make a dual 12v portable amp?

Portable like a cell phone in a pocket?

Or portable like a laptop in a bag?

9v batteries are just 6-8 batteries wired in series internally.  The closest batteries on the market 12v or higher will probably be battery packs made for the RC market.

Something like NIMH: http://www.all-battery.com/nimhnicdpacks48v-60v.aspx

Li-Ion: http://www.all-battery.com/li-ionpopacks37v-37v.aspx

Li-Ion would use fewer cells and generally be more compact as each cell is ~3.7v so 4 cells should get you near 15v.  NiMH cells are ~1.2v so it takes a lot of them in series to get the voltage up.  The smaller cells are a little taller and fatter than a AA battery for reference. 

Use mobile phone li-lion batteries 6 or 8 to get a +-11.1v or 14.8v. You will need a special charger for them.

The Li-Ion packs from RC toys are good too, but they are a bit bigger and heavy (because of high discharge rates) then a mobile phone ones with the same capacity.

The PPA battery board provides at least +/-12V

Czek that out, it also runs the only amp worth running portably, the PPA!. Not to discount the M3, but I dont think it likes running "cool" enough to run on batteries.

if you use sensitive iem you may have too much volume at even 1 V peak-to-peak drive V

there are op amps designed for 3 V supply that work fine - most DAP are limited by the small size of their output DC blocking Cap due to wanting smaller player size - not really that much by the inherent quality of available low voltage op amps

but you will need smt building skills - many newer op amps are only available in surface mount packaging

DC-DC converters are electrically noisey and efficiency at low powers is hard to achieve, only a few portable headphone amps have used them to boost battery V