Question for DAC/Speaker users.

I did a little searching and I don't see any USB DACs with dual line-level output.  There are some with multiple RCA input, but not the other way around.

I'm assuming you don't want to use the headphone out from a USB DAC/amp as an input to your amp - and that you don't want to skip your amp for the built-in one in a USB DAC/amp.  Otherwise, there's a lot of choices that have RCA line-level output and a headphone amp (the NuForce uDAC is one).

It's too bad your amp or speakers don't have a pass-through jack.  I know some others do, like the Beyerdynamic A1.

You could use a passive preamp (switching box) - they are usually overpriced, but this would be a really cool (and easy) DIY project.  That'd be your best a cheap option.

EDIT: Guess I missed the ones that do have dual output.

Consider a pro-am audio interface which offers multiple stereo channel input and output. 

Good regularly recommended products start with something like the Focusrite Saffire 6 @ ~$200.

http://www.focusrite.com/products/saffire/saffire_6_usb 

Up to the cream of the crop RME UFX @ ~$2,000

http://www.rme-audio.de/en_products_fireface_ufx.php

There are brilliant products at every price point between the two. As well as RME and Focusrite also look out for Presonus, TC-Konnekt, Apogee and E-MU among others. If you are really skint M-Audio and even Berhinger products will do a decent job for you before they break down.

you need something like Meier Stagedac.

it has two sets of RCA outputs.

one of those sets is tied to a variable output (volume knob).  that set adjusts the volume of the speakers.

when you want to listen to speakers, just turn up the volume.

when you don't, turn it all the way down.

simple as that.

as far as i know, there are very few dacs with these options out there......

Um, you have one that you're selling. You could just use your Fun as a DAC/preamp if you want to bypass its amp in favor of the Burson.

To bypass the Fun's amp you could go line level DAC Out of the Fun into the Burson (fixed level so volume needs to be controlled on the Burson) and preamp Line Out of the Fun to your active monitors (variable level with volume controlled on the Fun).

There are a few DACs that have an integrated headphone amp in the DAC (Lavry, Benchmark, and Grace come to mind, I know there are others.

Some DACs have both balanced and single ended outputs.  Easy to split the outputs.  I use this in my office setup - balanced to headphone amp, and single ended to speakers. 

The wyred4sound DAC2 will function as a pre-amp and has multiple outputs.  The DAC1 has as well, although not as many outputs.

Just some ideas.  Many other ideas out there - passive attenuators, and switch boxes etc.

Just looked at this review, which would be an easy way to go :

http://www.head-fi.org/forum/thread/526998/burson-ha-160d-amp-dac-unboxing-first-impressions

Can't comment on the NFB-3 other than to point out you'd be doing away with a preamp. If you were to go with the Fun again (which would probably still provide the best performance to cost option), I'd suggest you connect the Burson via Line Out instead, as passing the signal through the Fun's ACSS stage apparently has a positive affect on the SQ (according to Kingwa, and as stated by slim.a in his review). Line level DAC Out to your monitors would therefore require that volume be controlled on your speakers.