bounder,
Every multi-effects unit will work better in the loop where they are designed to go, as opposed to before the amp's preamp, where they are not. The only effects that should be placed in front of your amp's preamp are those that affect the core tone by boosting, overdriving, comb filtering, etc. Everything else is designed to come after the core of your tone has been established. This includes modulation effects such as flanger, chorus, phaser, etc., then delay effects such as echo or delay, then reverb effects.
Now, as to whether the multi-effects unit will sound better, it depends on which stomp boxes it would be replacing. Generally speaking, in terms of sound quality alone, no multi-effects unit can touch the equivalent boutique analog version of a given effect. However, if the stomp boxes you have now are all off-the-shelf unmodded ones, then a multi-effects unit might very well be able to compete.
