BassDuex, the 12AX7 tube or valve in the EDB-2 VAP can be switched in or out and can be applied as one side of it to each channel or can be "doubled" with both sides on Channel one for greater effect or saturation. The Gain control pot and valve/tube Balance control pot both increase and reduce the levels going through the valve. The Balance reflects the amount of signal going through, or bypassing it. You may prefer the single sided tube sound, which is lighter and clearer and in my opinion, realistic. The Doubler or extreme Balance settings, makes it thicker.
All tubes/ valves distort if a strong enough signal is passed through them. This unusual wiring for acoustic instruments here means that within reason, a stronger signal from the instrument pickup can be backed off using Gain and/or Balance controls to prevent distortion. Of course you might like distortion on occasions, although this setup has been mainly designed to reduce that.
12AX7 valves/tubes may be purchased from any decent guitar shop and some specialist electronics retailers. Different manufacturers tubes sound different. You want a low noise version, not the cheapest probably Chinese versions, as these can be microphonic and probably will not last long. The better Chinese versions are fine. There has been a shortage of valves, following the lock downs and various supply chain issues ( globalisation is going into reverse & Brexit is rubbish).
Since the tube/ valve can be switched off, you can still do the gig if it fails. Valves / Tubes should be seen as consumables, a little like light bulbs. They used to be made in light bulb factories, such as in the old Soviet Union, before transistors caught on. The tube is powered by the DC PSU supplied and cannot be powered by optional batteries, we call backups. So if you lose or break the PSU, or can't plug the mains connector into a socket in an overseas country, you can still do the gig, as the batteries power all other facilities on EDB-2 VAP. We didn't put batteries into the EDB-2 VAP because if we had done, we would expect complaints that the valve wasn't doing anything. So that's one complaint that's been avoided. I am happy to receive other complaints as that helps improve products. We are currently designing a 300w acoustic bass amp combo prototype using Class D, an aluminium housing and two drivers, a 6.5" 200w rms woofer and a 5.5" 120w co-axial, which handles a modest amount of treble and both at full power down to 70Hz but useable down to 60Hz. It will be well featured including two channels, tweeter defeat and probably a low noise compressor. It will be incredibly small, about 12-13 Litres for the sound cab and an extra litre for electronics with an external PSU. Modelling suggests it's capable of 120dB. John of Headway