Stereo speakers as subwoofer

My stereo system and home cinema system are completely separate but they share the same room. Because I watch movies less than once a month the cinema speakers (5.0 with no subwoofer) normally stored away are assembled only as and when needed. The home cinema system is very basic (Pioneer AV receiver + ProAc Response 1S's (normally as part of my second system in the study) + Castle compact centre and surround). Last night I fed the AV receiver's subwoofer output to my Naim stereo stack so that the stereo speakers (ProAc D30R) served the purpose of a subwoofer. As you might have expected the bass was not floor shaking but there was definitely more oomph. Of course a subwoofer is the proper way to go but being so heavy I cannot see myself moving it around like I do with the other speakers. Being a minimalist, a subwoofer sitting permanently in the lounge is not acceptable.

My question is will I damage the D30R by feeding them subwoofer signal? I could see the mid-bass driver cones moved a lot more than when playing music. The subwoofer output was set to -10dB just to play safe.