Changing Tubes

There are many great tubes in the 88 family but these four would be sonically my first choice.

Voskhods 6n23p 1975-1977 period (best bass, flow, effortless, extended, holographic), my best choice and the greatest value, fit and forget.

Siemens E188cc A frames 1970-1973 period (more detail than cca).

Reflektor 6n23p 1974-1975 period, most likely at the top of 88 family of tubes. Very expensive (the only one i do not own but have listened).

Amperex Holland ecc88 (Bugle boys) and e88cc D-getters 1958-1959 period.

The first three extremely quiet, very difficult to go microphonic, linear tubes and close to character compared with your EH but way much better. Reason for noting close to character is that your pre was created with such sounding tubes in mind.

Amperex are very transparent, extended and especially the ecc88 has great mids but can go microphonic.

More air and bigger soundstage if possible 

bigger soundstage = siemens, voskhods, reflektor

More air = amperex, raytheon, french made 88, tesla, telefunken

Above are only my general observations and as  i am not familiar with your preamp you may find something that fits your requirements better.

When a tube is upset mechanically by external vibrations and those are passed into signal. Due to construction most signal tubes are inherently microphonic but their location on a board can expose this phenomenon further.

Effects are feedback, ringing, hiss, rattle noises, static noises, distortion, high pitch noise, anomalies in frequency's response.