I have had the same problem only once with a preamp tube crackling when I hit the strings hard.
It sounds to me to be a bad, or microphonic preamp tube, if it only happens on channel one.
With Diezel amps, you will find that the company type ( such as Mullard, Telefunken, et.al.) and tube type (12AX7, 12AT7, 12AU7, et.al.) that you change out in a given tube, will allow you to voice the amp specifically to your personal tastes.
This is also true for the power tubes you decide to try.
I have an Einstein 50 watt combo and have done tube changing until I got my amp voiced for my needs. You are getting a really fine amp. Don't look at it as a lower end Diezel amp. There is no such thing with Diezel amps. The wattage requirements you need and the saturation types you want are all that matters with Diezel amps. They are all terrific.
50 watts is a great wattage as you will be able to run the tubes harder then a 100 watt amp and really get from the amp all you want from it. For extra volume, simply add additional speaker cabs. You will have a fine amp that will give you versatility and great tone!
If you decide to buy the amp, and after finding out that the crackling problem is simply a bad tube, start thinking about replacing the power tubes and preamps tubes with high quality types sooner or later. If the original owner used this amp a lot, the tubes might start going on you. I am not trying to cause alarm here, but look at it as buying a car with tires that need replacing when everything else about the car is working perfectly.
Read up on various sites and speak with people to find out which brand of tubes offer the tones you may have in your head. In the end, the better the tubes you use, the less problems with them you will most likely encounter, generally speaking. Remember that as with anything, sometimes the expected life of a given tube, no matter how high quality it is, may be shorter then expected. These things cannot be determined ahead of time. Keep spares just in case.
