Why do so few low power amplifiers using 300B have balanced inputs?

^^ He can, but 5 meters is a long ways to go single-ended, even if the cable is balanced. The adapter, if properly wired, will tie pin 3 to ground. At that point the cable capacitance will increase.

But with a simple mod (which is done by adding a coupling capacitor into the input tube's cathode circuit, which is tied to pin 3 of the XLR, pin 2 being tied to the RCA signal connection) then the balanced preamp can drive the amp directly with no need for an adapter.

The amp at that point will be operating with a differential input. It won't be balanced, but it will work better than with single-ended.

Depends on the adapter. ;) Putting an active stage or transformer at the amp end will do a pretty good job.  However, are yo sure there's something that needs fixing? 5m might be fine with shielded, single ended cables. Maybe. Worth trying first.

If you need transformer coupling, look up Jensen transformers, they have some great devices to do this for you.

Best,

Erik

A truly balanced circuit is like having 2 completely different circuits ....one for the + side, and one for the - side. This is a very expensive piece to build. Tube circuits are expensive to begin with....this would about double the cost. Designers aren’t sure the upgrade is worth the cost on the battlefield of sales.

I share the same thought stringreen,
Those low-powered 300b single tube per channel amps can’t by any means be fully balanced. The can however have as previously mentioned a differential input stage.