Can I hook up sub directly to amp?

It depends. How about telling us exactly the names and models of the devices you want to connect?

Kal

REL and some other subs can be connected to the amp. An alternative is to use Y connectors from the CD player, which I assume has a volume control, to both the sub and the amp. This can work quite well, the critical factor will be the quality of the output circuitry of the CD player, they are not all up to the job. Your CD player could have no inputs, its only function is to read CDs. The only exceptions would be ones that allow you to use their internal decoder with an external transport, but this would be found on very high end players only or possibly some universal players.

Kal
I do not understand about Y connectors and the amp.
The cd player would be a Bel Canto CD2 which has no inputs but has volume control. That is why I wonder if you can just hook up wire from sub to binding posts of my AR 100.2 amp.
Thanks as I am getting different answers.

If your sub has a low pass filter and you do not want to run your mains full range, you should be able to go CD OUTput (not input) to sub to amp. Also many subs can run from the high level speaker outputs on your amp (essentially in parallel w your mains). Still haven't told us what sub you have. Also Vandersteen 2W and 2X can only run in parallel w your mains. There is a filter (aka crossover) that (in your case) would go between the CDP output and the amp input, and then both the mains and the sub get a speaker level connection to your ARC.

What sub?