Even though I can see a couple of reasons to NOT use it as a subwoofer amp The above is actually not bad advise. (So I modified my previous post slightly) Depending on your use case the flaws might not be a total showstopper, but for serious HT use its going to suck real bad
Now that you have it, at least try it. You might like the punchy sound from the additional distortion, 50Hz peaking and rolled of last octave
On the bright side it does have an (although non defeatable and maybe not 100% intentional) Infrasonic filter which might work well with some ported subs with limited excursion capability .
Good to use with vinyl.