This is an older thread, but just to add my experience, the tone change isn't massive.
I used a different preamp and pickup set. You get more tonal options because of the EQ shaping, but outside of that the tone isn't drastically different. I wouldn't say it's worth the money in hindsight.
Assuming you have the other electronics that come with your pickups, you can get the BTS and Mid Stack for less money add get most of the features. Both preamps are pretty clean with EMG pickups. You won't have the tone control that you would get with the John East pre, so you'll have to decide if that is worth it. You also won't have the bypass switch, which in an already active bass... isn't the most used thing in my experience. In a Non EMG bass my opinion would be different. I will say I have a lot better experience in installing the EMG. Both are "solderless", but the EMG is a much easier solder less system especially with already existing EMG pickups. Last, and the most minor thing is the knobs for the EMG are trash. Like absolute trash. Plastic and they didn't tighten well. John East comes with premium quality knobs, which I would have never valued before having them. Now I buy expensive knobs for all of my basses.
My experience is pairing the Victor Wooten P/J set with the pre uni 5 and also pairing the EMGs with the BTC and Mid control (VMC). I have also paired the BTC and VMC with non-EMG pickups. It is an extremely clean preamp. I wish I had the experience of pairing the John East with my Non-EMG pickups, but installation of preamps is a pain and I will not do it again lol