Subwoofers: speaker level vs line level

Although my sub accepts both line level and speaker level input, I have only ever used preamp's unbalanced line level out to sub as all speaker terminals are occupied with bi-wiring the main towers via banana connectors. I know that REL, maybe others, recommend speaker level connection, which I wouldn't mind trying (not that I'm displeased with present setup). Would there be some way of doing this other than bridging the speaker inputs thereby freeing up terminals to run subs? Or, if continuing to bi-wire, switching to spade connectors for towers while using bananas to sub? (Amp does offer loop out, but that is line level). Assuming these approaches would work, would there be enough benefit to justify the hassle?