subwoofer speaker cables under the house

FYI, IMHO you should have both lengths equal. Especially at those distances. Shorter lengths would not matter so much.

Have you tried Blue Jeans Cables?

relatively cheap.

You might get by using a good RG6 cable like Belden 1694 terminated with RCA's.But don't hold me to it.Check the web.
A link for some talk about it.Link>>>>>http://forums.audioholics.com/forums/showthread.php?t=13245

Link>>>[http://forums.audioholics.com/forums/showthread.php?t=13245]

Also don't run any interconnects parallel with AC power lines.

I forgot to mention the main reason I suggested the Belden
1694 is because it uses a copper center, and sheild too I
think.1694a is a solid copper center,1694f has a stranded or
more flexible system.Regular RG6 is mostly copper covered
solid steel center,and aluminum shielding.Not the best,but
gets by.A friend uses RG6 for 6'interconnects,and says it
works great.I haven't heard it though.The dealers might be
of help about RG6 for sub-interconnects.Maybe while your at
it,think about CAT5e- CAT6 cable runs for future something?
A link for 1694 cable.>>>[http://www.belden.com/pdfs/Prodbull/NP233.pdf] Belden makes all kinds of a/v wire.

SORRY! I misread your question.I was thinking line level.
Signed Embarrassed!!!Maybe 12 gauge CL3 for in wall
use.Whatever brand,I think it needs the CL3 rating for in
wall use.

I second the Blue Jeans cable.

mitcables.com/available-in-stores/custom-installation

i use this stuff for my surround long runs inwall.

14 gauge minimum should do it - 12 gauge if you are fanatical.

I third Blue Jean cables - listen to this advice as this is a no - brainer. Killer sub cable for very cheap.

Cost per value blue jeans cables. 12 or 14 guage. If you want to go a little high Kimber makes some nice 12/4 speaker cable I run alot of. This is a must - Keep the speaker cables the same length though. Changing the length effects the sound a little so you want both speaker wiires at 40' so they have the same resistance etc.

which blue jeans speaker cable were you guys recommending?

Straightwire has a some great affordable options. 
 

Super Quad 12/4 (SQ4):  The first CL rated cable specifically designed for today's demanding high performance speaker systems. Four 12 gauge conductors of OFC copper with advanced polypropylene insulation, CL 2 rated. Power and control for maximum performance even over long lengths. Product of the Year 2003 - CE Pro magazine.

Putty jacket
9.5mm (.39") diameter  

SPECIALTY SPEAKER CABLES