@freediver Try the Benchmark stack on it, AHB2 amp | LA4 preamp | DAC3B dac. You can always try this on a home trial or take a chance and buy used.
With warm speakers this stack would open up the sound. Doesn't the Decco have a tube in it. Warm speakers with a tube is not my type of system.
A cheaper option is a Class D like the PeachTree GAN400. I am putting my up for sale on USAM.
Put a streamer into the path instead of going direct via USB from the laptop. Not so much for your issue but to improve everything in the sound.
I have not heard the Decco but owned the PeachTree Nova 150. That is terrible compared to the new Peachtree gear such as the GAN400 and better yet, the GAN1 (but should modded it by tweakaudio). I replaced $15K of gear wit the $1200 GAN1 + $600 mods. However, that unit can ONLY stream via SPDIF, no analogue sources allowed. I used ROON to both stream to that and control volume. I replaced my Benchmark stack with the GAN1 since it was similar for lower cost, though eventually the lack of analogue inputs made me sideline the GAN1 only for headphones.