"I have a 10 year old Marshall tube amp with an effects loop. I want to use my multi-effects processor with it, but it seems like it kills the sound of the amp when I hook it up. Doesn't matter if I plug in between the guitar and amp or use the effects loop. Is there any way for me to use it?"

This is a very misunderstood subject so we're glad you asked. Your problems stem primarily from level mismatches. The input of your amp and your multi-effects unit both want to have the guitar plugged directly into them, and they both want to have their outputs sent directly to a power amp, which in turn drives the speakers.

The solutions for this vary depending upon exactly what you are trying to accomplish with your effects unit, and how flexible your effects box is. If you have a high quality effects processor and you are primarily wanting to use it for time-based effects (not distortions and preamps) then you should be able to get it working through the amp's effects loop (this is, in fact, what the effects loop is for). It may take some experimenting with the input and output levels on the effect, but it should ultimately work fine. Many players get in to trouble here by using effects that can't provide enough of a return signal to the guitar amp to adequately drive its power amp, resulting in a wimpy sound. A small guitar floor type pedal (stomp box) isn't going to cut it. Those have to go between the guitar and the input of the amp (generally). The effect send of many of these amps is a relatively low level -10 dBV signal so make sure you crank up the gain to the effect enough to drive it properly, which shouldn't be an issue with most boxes. If your effect unit only has balanced +4 dBu inputs you should add a level matching interface in between. (For more than you ever wanted to know about +4 dBu versus -10 dBV check out our dB reference in the Summits area of inSync.)

If the above method doesn't work effectively for you, or if you want to use your multi-effect unit for things like distortion, overdrive, compression, gating and other dynamics processing in addition to time-based effects, you'll need a more elaborate hook up. In order to get there you need to make sure your effects unit has its own send and return capability. Most modern high-quality units that include this variety of effects do. Hook it up to your amp as follows:

Plug your guitar into the input of the multi-effects unit.
Plug the mono out of the multi-effects unit into your amp's effect loop return.
Plug the send of the effects box into the input of the amp (where you'd normally connect your guitar).
Plug the effect loop send of the amp into the multi-effects unit's effect loop return.
If the effects loop of the multi-effect unit can be switched on or off make sure you have it switched on.

This is a surprisingly underutilized, misunderstood, and very effective method of routing your signal with many guitar amps. In this scenario your guitar's signal goes to your effects unit, then is sent from the send of that into the input of your guitar amp. From there your guitar amp's effect send routs the signal back to the return of the effect unit. Finally the output of your effect unit is returned to the amp through its loop return as normal. This segregates the distortion type effects of the effect unit from the time-based effects, which normally occur after the send/return loop. Basically you're putting the preamp stage of your guitar amp in between. Applying these time based effects after the preamp and distortion stages of both devices can make a huge difference in your overall sound. Normally you'd use the clean channel of your amp (or set it up to make a clean sound) when you are using the distortions in the effects unit, but there's no hard rule for this. You can use a little of both. Try it.

If you still can't get a satisfactory sound you may be one of the thousands of guitarists who simply shouldn't use effects connected directly to your amp (or you'll need to switch to an amp that's better at accommodating them). You'll need to find a way to apply them afterwards - a subject we'll cover in a future edition of inSync.