Not sure how much you paid for the 501 or what your budget is overall but the 501 has been replaced by the 503 and it seems to have a volume control. Here is one for sale here:
https://www.audiogon.com/listings/da-converters-teac-ud-503-dac-peramp-2016-09-17-digital-n2z1x4
It might be the better long term move versus buying the 301 and maybe deciding a year or two from now you want to upgrade it.
https://www.audiogon.com/listings/da-converters-teac-ud-503-dac-peramp-2016-09-17-digital-n2z1x4
It might be the better long term move versus buying the 301 and maybe deciding a year or two from now you want to upgrade it.