What are the advantages/disadvantages/difference of using a solid state, discrete output stage instead of valves?

In the aft end of our circuit we have our own reasons.
Most of it revolves around keeping the circuitry tight for good transient response and detail. Our output buffer runs on a +60 volt rail for a wide dynamic range and low noise floor.The valves themselves run on a +300 volt rail.
Textbook and theorists may differ in the sonic advantages here ,, of course the human ear should have the final say and differs from soul to soul.
Basically the vmp output stage is configured for transparency so that the only circuitry colour encountered is from the input transformer and valve gain stages.

And what are the differences sonically?

We find that we have increased bandwidth and dynamic range in the aft section of the circuit by using that solid state buffer. Transient response and detail are maintained and preserved rather than smeared by having it go through a coil once again.
Also , sonically the response has a tendency to be more linear .
In other words , the unit performs the same at 2 on the dial as it does on 8.
A transformer coupling stage at the end of the circuit would sing differently according to the signal entering it.

What is the advantage/disadvantage/difference of having both input and output gain controls (most tube pres) and the Seb?

Sometimes to record high SPL sounds ( drums, brass … some vocals ) it is preferable to have just one short tight gain stage to preserve signal purity.
Using the input and out put configuration setup you need an extra gain stage to make up the gain loss because of the extra potentiometer/attenuator.

If the circuit has an additional gain stage it’s a drag because it means you’re just attenuating the signal down after you’ve already amplified it ,, this is lost energy and can also be the source of circuit noise ,,, though not apparent immediately but with tracking or layering it all adds up….
However , if you listen closely or run the right sort of tests , it’s that makeup gain stage that usually fouls up sounds because of it’s own artefacts and restrictions.
So when you compare the difference to a tight gain setup , the tight setup is most preferable because of less artefacts and circuit crap.
The extension of this is most pro-sumer equipment that may have many op-amps in an audio path….The shortest distance between two points is always a straight line.

What functional and sonic difference is there between using the traditional input/output controls and cascading channels with a Seb?

It depends on the configuration of the circuit you are comparing it to.
Dynamics can be controlled in many ways within a circuit. Attenuation through a shunt level or volume control is just one way…. Positive and Negative feedback are another way….
Positive feedback : Sending a positive phase signal back to the input to increase gain ( level or volume ) of a particular signal but will increase amplifying artefacts such as distortion or noise etc…..This increases the colour.
Negative feedback: Sending an inverse ( 180 degrees out of phase ) signal back to the input of a particular gain stage will reduce gain ( level or volume ) of a particular signal as well as cancelling out most amplifying artefacts such as distortion and noise….increasing negative feedback roughly transfers to increasing transparency or decreasing colour …..

Besides all this …., properly biased valves don’t always offer the best symmetry for the human ear.

All these variables shape the wave differently when the valve has more juice than usual.Mind you , sometimes the differences are negligible and are sometimes not worth fussing over…

With the Sebatron VMP configuration ,, negative feedback is controlled by the pad switch and output level drives the output buffer.The first gain stage of the triode is assigned to the pad control and feedback is controlled here… allowing the unit to be run transparent to coloured.

For an extra kick in the guts , hooking up the two channels allows the user to adjust the output level of the first valve feeding into the second valve with selective feedback control via the pad switches.This basically means you have a much broader range of colour options than just the two dials running the input and output levels.

Engineers differ from what they want in a preamp.Some prefer one dial because that’s all a preamp should do … amplify.
If you have two dials you may hit a cool sound but find it hard to recall because of the continuous nature of a pot control.That’s why we use a three position selector on the pad control and why I’ve designed the colour control of the THORAX to have six steps.

GooGOoCajOOb 8)

Wow, great reply Mr. Seb. I will now attempt to digest.

This reply just sold me on a Sebatron unit. Not because of the rave reviews, and half of the stuff said I don't fully understand. It's the fact that you took the time to explain this in detail the best way you could.

It shows a lot about a company when they are signed up in a third-party forum to answer individual questions from potential and existing customers. You don't get this quality of service from Groove Tubes, Neve, API, or anyone else!

So that being said, I just wanted to say that I think your company already kicks ass and I haven't even heard your product yet (a little out of budget because I want a 4 channel),

Thanks Arthur . I appreciate it !

Thats exacly what i was gunna say Seb :lol