Help wanted spending money and improving my system.

Get a High Fidelity Cables Double Helix Signature power conditioner just added one to my system wow what a difference. Simply plugged it into my power distributor so it treats all components. They're even on sale right now, but the difference was incredible.

The 2 most obvious choices for me would be (yes, I know the OP only asked for one).
1) replace the DAC with a Bryston BDP3 which has HDMI inputs.  Then program the Oppo to output DSD over HDMI and you can enjoy the DSD layer of your SACDs.  The Bryston is also a better DAC than what you currently have and so this upgrade will be the most significant.
2)replacing the Mac Mini is for me an obvious second choice

Hi,

Given that your system is prey much done, I would invest on power equipment.

Would start with a IFI AC Purifier or a Isotek AC conditioner.

Filipe

Get a better TT/arm and cartridge. The new $1800 Technics would be a significant upgrade.

Dream question.  Lovely system, by the way. A number of the components have been in my wish list at one time or another!
I have found that the new tech tweaks will improve a system far more than changing components.  I will list the progression where I have found the most dramatic changes in my path, ending in delight (for the time being). You can stop anywhere - you said only one change, but I suspect you wont, if you do decide to go this route.

1) HFT's are brilliant.  I have 30-40 in my room.  The first 5 give a taste of where they are taking your room...However, you will still need to diffuse early reflection points no matter how many you have. There are ten of these points, unless the reflected signal path is 17 or more feet longer than the direct signal.  If appearance is not a constraint, then use the new cheap foam diffusers, less than a couple 100 for all 10.  

2) Good component footers. I have tried everything under the sun.  The ones that stand out in that they embody a new concept - drain entropy from your system (whatever that means) are the centre stage footers from Critical Mass Systems.  Forget about anything else no matter the price (my previous ones cost 5 times what these cost).   Check reviews.  Suggest buy one set (CS2's are available in the used market, the new CS2M's are not) then go running back for more.  Base price for one set of 4 CS2M, is around 1000/-. 

3) Switches.  Install reclocking switches in your digital chain as close to the DAC as possible.  I have a cascade of 3 top ones, and each was a new adventure.... Uptone audio make the EtherRegen, great product. Cheapest and among the best money can buy in terms of Sound Quality.   If you install more than one, learn about fibre connecters. They blow away copper.  A good resource to learn is afterdark.com. Start at 650/- for a single ER, other companies much more expensive for equivalent quality.

4) External clocks.  These make magic but you have to have the infrastructure.  Again best resource to learn and buy is Afterdark.

This progression has brought me to where I am not currently lusting after my next upgrade.  

Hope this was helpful.

Please read my earlier 2 messages again after this, if these suggestions interest you.
Best wishes
Aubrey