balanced preamp

Going internally balanced is a good way to reduce distortion of the circuit. This is true whether the preamp is tube or solid state.

If the balanced operation is done properly, the interconnect cables cease to be a part of the sound of the system, unlike single-ended. Note the caveat 'done properly'.

Kijanki, you are correct about the distortion product. However the less distortion is made the better. If you do the design right, the primary harmonic to show up will be the 3rd. If you don't, you'll have a mess on your hands :)

I don't find in practice that cost is much of an issue. If you are to build a world-class preamp, it will have to have good power supplies whether it is balanced or single-ended. All the things you have to do to make a single-ended preamp really work right cost about the same.

In practice I've not found your comment about gains being perfectly matched to be correct. I've seen minor differences that show up if you have unmatched tubes and the like, yet the CMRR is still quite good (there is a lot to be said for a good CCS design!). If the preamp were balanced but not differential then you might have a point.

Kijanki, we don't have any output transformers so we don't have that expense anyway...

Is it me or is 85 db a poor CMRR figure?

Like I said a lot depends on an effective CCS circuit. I have to say I am quite surprised to see how poor they are in most differential circuits I see. Poor CCS leads to poor CMRR response! You can't do it with a single-stage CCS...

Not only cost of tubes but they also require more power.

Where are you going with that?? That does not have anything to do with CMRR... and is not accurate. The tubes we use are not expensive, and usually cost less than a set of tubes for a transformer-coupled amp of the same power.

From your other comments though it really sounds like the circuits you are used to measuring have poor CCS circuits. 85 db is not a hard figure to attain.

OK Kijanki, I will take the other comments as a red herring then.

From your comments it seems to me that you are used to working with balanced gear that uses the ground connection. The ground is supposed to be ignored. But a lot of balanced gear uses it, which degrades their CMRR. I see this all the time in high end. Unfortunately, lots of instrumentation amplifiers have this same design flaw.

However, single ended preamps have no distortion cancelling capacity, and nothing for a CMRR. This is not saying that they can't sound good by any means, but it does mean that they are prone to interactions with the interconnect cable, something that should not happen with balanced operation.