DAC suggestion Please

RME makes some very good stuff if you are looking at the mid-proaudio level. It is a distributed and "reseller" item, so there will be considerable mark-up. That’s not bad, but expected with many item. There are some things that the Gustard has which the RME does not have:

- Two separate fully linear power supplies
- Multiple crystal clocks for different sampling rates (i.e. 44.1/88.2/172.4 vs 48/96/192). Extremely high quality clock for main system CPU.
- Fully discrete analog output sections (that are balanced as well)
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While the RME is not bad, it uses a wall-wart swtiching power supply, in addition to several other things. I’m sure it will work fine, but I think the Gustard is definitely on another level.

Check out Denafrips. 
They have 4 dacs in thier lineup. Suitable for any budget.
They are discrete R2R dacs, sound much better than DACs with delta sigma 'chips'..The tonality is some of the best I heard. I personally own the terminator, thier flagship.
To explain the caliber, Just yesterday we had the chance to compare my terminator vs a chord Dave, along with couple of friends. I could assure you that the owner of the Dave was not pleased at all :)) the terminator bested it on every aspect!!
You don't have to go to the terminator if it was out of your budget. The sound signature is almost same along the range. Things get more refined as you go up the ladder, bigger sound stage with bigger power supplies etc, according to what I read. But with my habit of upgrading and wanting the best (with reasonable financial limits!!), I went straight to the top to save myself from the losses when that urge kicks in. I think I'll have this DAC in my system for a very very long time.

You have a $300 pair of speakers that includes amplification.  To my mind, your Zdac already outclasses your speakers.  I think you'll get a much bigger improvement upgrading your speakers at this point.  Best of luck. 

Going up to LS50 actives would be a much better upgrade path, IMO.

I would say that upgrading either the active speakers OR the DAC will give an increase in sound quality.  The KRK Rockit are not bad, and they are at the same level as my Yamaha HS8 (only smaller).  I have been able to make my Yamaha monitors just "sing" by doing DAC/preamp upgrades as well as power cord/fuse upgrades.   Sure, the Kef LS50 are definitely more accurate and high resolution, but the DAC source is important as well.