for better or worse, a speaker change is going to have a much greater effect.
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The stock tube in my Decco was a Russian 6H23n and it's actually a pretty nice-sounding tube.
I tried most of the tubes I have around that would work in the Decco and really liked the Amperex 6DJ8 Orange Globe but it's one of a pair that I like to use in my tube amp so it's back there now. I listened to the Siemens 7308, Tesla E88CC, Ediswan CV2492 and Ediswan PCC88 and didn't come away with a clear favorite. The Tesla is in there now and it sounds wonderful but it may just be that I have an affection for old Tesla tubes.
I would suggest you get in touch with a really knowledgeable dealer like Jim McShane and buy two or three of his recommended tubes in the 6DJ8/6922/7308/E88CC family. It won't cost a lot and you'd have the fun of swapping them out and hearing the differences.
Oh, and as long as you've got the Decco open and are swapping tubes there's a small change I would suggest, at the risk of being ridiculed for even bothering with it.
About the only thing I didn't like about the Decco was the LED they install in the tube socket to 'enhance' the tube glow. Besides being silly, they use a bright yellow LED that's nowhere near the color of a glowing tube and I thought the effect was more 'pin-ball machine' than 'tube preamp.'
While I was trying out different tubes I gave the LED about three coats of dark red fingernail polish, figuring the bright yellow and dark red might together equal orange. It worked perfectly, it's exactly the same color as the glow in the top of the tube and the intensity is just right. Besides, now I have a reason to laugh at myself every time I use the Decco.