How to add a subwoofer to a 2 channel stereo

If the sound is similar at the same volume levels when using either amp, the problem is the subwoofer.  If the sound is poor out of the dedicated at the same volume levels, the problem is the subwoofer amp.

Location and phase are also issues, but I assume those are the same with both amps.

Also keep in mind, depending on how the subwoofer is constructed, if you place a cinder block or two on top (wrapped in a towel or some material), it will tighten up the sound quite a bit.  Cinder blocks only cost a few dollars to try it.

Thanks for the reply. My issue is, the sub sounds less clear if I use dedicated sub amp and split the signal from preamp to amp. However, if I just use one amp, and bi-wire to speaker and this passive subwoofer, the subwoofer is clearer but has lower volume, ie lossed the "punch". 

This old Mcintosh HT2 has two 12 inch subwoofers. Can I add another binding post and turn this mono due subwoofer to a stereo subwoofer? By this way, I may resolve the issue. Any suggestions?