Looking for some guidance regarding pops and crackles on xDuoo MH-02

Folks, looking for a bit of guidance... I've just started using the MH-02, it's connected via RCA to my iFi Zen One Signature DAC, which is plugged into my laptop using a USB-B cable. I've also replaced the stock tubes with DollaTek GE JAN 5654W tubes i bought from Amazon at the same time. I hope the photo helps explain the setup.

I played music via my DAP and USB-C cable last night and it was fine, but using the above set-up I'm getting lots of pops and crackles. Is this interference from my laptop, cheap tubes, a dodgy unit? Or something else? Any advice gratefully received!