I have my DAC hooked directly to power amps, so I'm in the situation you describe. I would advise doing the following:
1) Determine how much power your speakers can safely accept (RMS and peak). Then determine the gain of your amplifier and how much power your amp is capable of outputting. Then determine the maximum line voltage output from your DAC and figure out how much power that would translate to after going through your amp. If your amps have volume controls, set the volume control so that if the DAC outputs maximum line voltage the power reaching your speakers is at or below their RMS power handling level. If your amps do not have volume controls (like mine), consider putting passive line level attenuators between your DAC and amp to bring the maximum output down to a safer level. Once you've done this, you can be assured that even if the computer outputs maximum volume, it is very unlikely to damage your speakers (your ears are another matter, but we'll take additional precautions below).
2) Set the maximum volume in JRiver to the maximum level you'd ever want to listen to music or movies. Set it good and loud, but not so loud as to be painful, and set the maximum there. Under ordinary circumstances sound will never get louder than that threshold.
3) Set the default windows sound device either to 1) JRiver's WDM driver, which will be restrain sound to the maximum volume level set in 2), or set the default windows device to a disconnected sound device. I just used my onboard audio for this before the WDM driver came around, but the WDM driver works well for this.
I've been operating with nothing but a passive attenuator between my DAC and block amps for more than 3 years, and I've only had two "oops" blowouts; one from using a not particularly well-designed 3rd party VST plugin and one from inadvertantly disabling the volume maximum (i.e. both were user error). My speakers were fine because I took the precautions in 1).
I can recommend it; every active component in the path is a guaranteed source of some amount of distortion, and JRiver's volume control is very reliable.