no volume control

How are you currently controlling volume? I'm guessing through your computer's sound card or your music playback software, but please clarify. If that't the case, use the DACs RCA or XLR output to drive a headphone amp, which typically has a volume control. The software based volume controls in the computer typically cause a loss of resolution if you lower the volume more than a few dB, so that is not an optimal solution. Which headphone amp is dependent on your budget and what kind of headphones you are using. If your two channel rotel actually is an integrated amp w a headphone jack, take the DAC's output to any rotel input and then plug into headphone jack. There are converts that will allow a conventional power amplifier to drive headphones, but they cost about the same as a moderate quality headphone amp, many of which are battery operated and can be with your streaming device and headphones on the go.

OK, now I get it. so my recommendation would be USB out to DAC. DAC output to passive pre. since it does not appear that you need a source selector, I would recommend the Lightspeed Attenuator which you can buy from member Georgelofi. do a search to read the long thread about the LSA which is considered to be one of the true high end bargains out there. I've had one in my system while by main pre was out being upgraded. Cost difference about 20X, sound quality difference not all that much.