Actually very good. The main reason I upgraded was because I upgraded my preamp. My old LS25 mk2 used a 15 amp power cord. The Ref 5se needs a 20 amp. So I needed a new power cord. I chose to go with the Ztron Alpha HC after talking with Grant from Shunyata. The Zitron Alpha HC is excellent. Better than the Black Mamba. It is even better than the Sigma Analog cable. I have since upgraded all my power cables to Zitron Alpha HC cables. I have heard that the new Delta is as good as the Alpha but I have not tried one.
The Black Mamba has a cult following so some people ask a lot for them. I would not pay over $350 for the HC. I paid $325 each for the 2 I had. If it were me I would hold out for an Alpha HC. Unless you have a Shunyata dealer near yu and can demo the Delta.
You can search Hifishark.com and have them send you email alerts. It won't hurt to get the feel of the Ref 5se with the stock cord. Then you can more easily appreciate the difference that the Alpha makes.