I had the 2100 in its day, but don't remember anything about the twin part. It may be just a reminder that it was stereo, instead of monaural. These were some of the earlier stereo amps. If you parallel the speakers on the 4 ohm tap, I don't see a problem. If I remember right, these amps run cool, so if it gets to warm, maybe your overdoing it. Don't forget they are old,60s to early 70s I think.Over 40 years old?. Might be the first stereo amp? I think it was one of their better ones. These amps were used (abused) by bands and discos. I hope you got a good one used in a home and it works out.
