Thanks for allowing our input. I'm a fan of the open euphonium choir version. IMO your solo has no problem cutting through. I'd be a fan of leaving out the piano entirely or at least reducing the volume of the piano. The Euph choir with solo is just so soothing and tranquil that the piano accompaniment, especially arpeggios could be a bit more delicately presented (just tickling the ivories).
Bob Tampa FL USA
Euph -- 1984 B&H Round Stamp Sovereign 967 / 1978 Besson NS 767 / Early 90s Sterling MP: 4AL and GW Carbonaria
Tuba -- 2014 Wisemann 900 CC / 2013 Mack 410 MP: Blokepiece Symphony American Shank and 33.2 #2 Rim