Recs for Tubes & tweaks for HH Scott separates

Have had LK 150 amp LC21 pre pair as 3rd system.  Love the midrange, dynamic range, and layering (front to back separation and instrument placement) of the sound stage that it does.

Disappointed in mushy base and rollEd off highs.  Was wondering if anyone had experience with these or similar amps and could recommend tubes or other tweaks to improve the sound.

Pre amp uses six 12A7X and one 6X4. Have 4 Sovtek and 1 telefunken + 1 Realistic holdover in now.

Amp uses two 7199 phase inverteters, four 6550  output,  and two 5AR4 rectifiers.  Have nos 7199 RCA and JJ 5AR4 in. Have new Russian Tung Sol 6550s in preferring them to SED 6550 and Sovtek KT 88s.

Have used primarily w budget Morrow MA1 interconnect and SP1 cables and budget bookshelf speakers like Athena BS1, Infinity Primus 143,  and Epos ELS8.

Thinking of trying new Russian mallard kt88 output tubes. Matched preamp tubes and upgraded cables also probably also help.

I'd appreciate any tube new or NOS or other recommendations that anyone out there might have.

 Thanks,
CN