I too have often found that ultimately beneficial system changes can sometimes fool you at first into thinking that you're 'losing something', which upon extended listening actually turns out to have been spurious artifacts.
Changing tubes can make as big or bigger a difference than the power cord. And - although this is not a cure for 'dark' tonal balance per se - I would also recommend trying some sort of dampers on the preamp tubes (if you don't already have them), such as Herbie's Audio Labs or the silicone ring types. This can bring a calming stability to the soundstage and reduce glare and definition loss on transients.
Changing tubes can make as big or bigger a difference than the power cord. And - although this is not a cure for 'dark' tonal balance per se - I would also recommend trying some sort of dampers on the preamp tubes (if you don't already have them), such as Herbie's Audio Labs or the silicone ring types. This can bring a calming stability to the soundstage and reduce glare and definition loss on transients.