bad stone output cap

[lowstar]

hi folks,
i´m tinkering some more with my bad stone in terms of bass friendliness.
http://topopiccione.atspace.com/pjimages/EHBadStone.sch.gif
the input cap/res network rolls off at 15.9Hz (if i´m right), that´s ok.
but how about the output ? (it is at point K in the schemo, upper/middle section of pic)
the way i understand it, it is just a coupling capacitor, or am i missing something in terms of R6+C1 or R5 and R28 having some influence ?

and if it´s just c15, i have searched but have not found a real formula, so i´d like to know just roughly which size cap i should pick if i want to let all bass freqs through, 0.01 seems a bit small to me ?

cheers,
lowstar