They only have a frequency bandwidth of 200 khz, so it can only take 200,000 samples per second.
With that bandwidth, I would only use it for signals up to 20 khz or so. So, IMHO, it would work for analog signals but not really for digital.
Oh, and although it says the voltage range is-14 to 20v, you can use a x10 probe making the voltage range -140 to 200v, which is fine for normal stuff, but not really enough for tubes.
Randy