Feedback can be a very difficult problem to deal with. I used to have all kinds of problems with that, even using only a piezo type pick-up.
Lately, I've gotten it down to a minor and infrequent aggravation using a very nice miniature condenser. Condensers are not generally worse than dynamics, IMO. It is just that most condensers are omi-directional and most common dynamic mics are cardioid (focused) pick-up pattern. As long as your mic is cardioid pick-up pattern it should perform as well as a cardioid dynamic mic.
Things I have found helpful:
Put the mic as close as possible to the top of the bass. Under the bridge or under the fingerboard works well. I haven't found that putting it out from the bass is better. It might work if you have an acre for a stage, but mostly you will just get all kind of other stuff and possibly room noise mixed in with your bass.
Do not run the bass signal though any stage monitors if you have those.
Put your speakers out in front of you if possible;- as Larry suggested, farther away is better.
Parametric EQ. Some amps have something like this built in and so do some PA systems. Essentially you find the offending frequency and cut a "notch" out to kill the feed back. On a graphic EQ you need several individual bands to do as well, but you can still get close if you have something like 10 bands;- 30 gives you much finer selectivity.
Phase reversal. This is the secret weapon. If you can get an amp or gadget that reverses the phase of the speakers from the phase of the vibrating bass top, this one little trick will knock out the worst big WOOOOM type of feed back.
There is an American made amp (probably difficult to get in Italy, but it would be worth trying), the Acoustic Image Coda R series III, that has many of these features included in such a way that they are very easy to use. It is sort of like a two channel PA within a combo cab (they also offer separate amp heads) that is extremely versatile and designed with amping DB in mind. If you really like using a microphone alone, I suggest you try to find one of these. For me it was the "silver bullet" against feedback.