Tip 1 - Put the Gate Up
This plug in can be very 'hot'. What I mean by that is with all the gain they put out – especially the E 646 model - there may be a lot of pickup noise associated with it. Luckily, it has a built-in Noise Gate (accessible by clicking the FX Rack button), which I find essential. Simply turn the Noise Gate switch 'up' to the ON position and push the THRESHOLD control up until you hear the buzzing stop. At that point, you'll see the 'CLOSED' LED light go on, indicating the Noise Gate CLOSED – or working! You'll need to play around with the THRESHOLD level to make sure you don't cut off any of the guitar signal you want to get through. I find the 12 o'clock position a great place to start. You should also play around with the RANGE control. This lets you dim the output level only by a little bit or a lot (measured in dBs or decibels). The setting on the RANGE control goes from 0dB (left) up to 100 dB (right). Like the THRESHOLD, I'll start around 12 o'clock (50 dB) and take it from there in either direction. Can't say enough how useful this built-in Gate is to keeping that buzz down.
Tip 2 - Filter Time
Another useful built-in effect on the FX Rack are the Amp Filters. Both are switchable from Off to either Pre or Post. Pre is where the filter is in before your guitar signal even hits the amp. Post is after the amp. I happen to like to do my filtering after the amp section, so tend to use Post. The Tight filter lets you cut LOW frequencies out of the sound. Selectable from 30 Hz (left to 3000 Hz (right), it will help you clean up the bottom of your guitar sound. If you've got a lot of bass in the mix, you'd be surprised how far up you can push this tight filter (try even up to 400 Hz). While the guitar may sound 'light' soloed, in the mix, it may sit just perfectly in the mix and help you clean up the entire low end. The same applies for the Smooth filter, which rolls off hi frequencies. This can range from 3000 Hz (left) up the 35,000 Hz (right). These ENGL amps can get a bit edgy, and using the Smooth knob is a nice way to roll off some of that 10K crispness without losing the treble.
Tip 3 - Echo, Echo, Echo
Aside of the useful built-in Tap delay, which is also accessible by opening the FX Rack, I often turn to such classic goodies as Universal Audios EP34 Tape Echo. Using a stereo AUX track, I assign the EP34 to it and use a bus send from my ENGL track to get signal to the plug in.
My go-to sound is a preset I've created call Ambience Panner. Using a very short (99 ms or so) Echo Delay setting, I pan the Echo to the far right side. Typically, I'll pan my ENGL amp to the opposite side (at about the 10 o'clock position). When you start to push the bus send up to the EP34, you'll hear the short echo ambience on the right side. Again, the farther you push the bus send, the more echo you'll have. Of course, you can have the guitar on the right and ambience on the left, etc. – whatever suits your needs. It's also worth playing around with the Echo Tone. The Treble and Bass knobs will help push your echo forward (by brightening it) or back (by darkening it) in your mix. Try experimenting with the Echo Repeats, because if you push the knob up to say the 12 o'clock position, it will go into a really cool 'self oscillation' mode. Its kind of trippy but can be a great production technique.
Tip 4 - Make enough room
The thing about guitar plug-ins is that they are one dimensional, unless you add some echoes or delay as mentioned above. Another technique that works great on these amps is to use some room sound. While you can use any room simulator, I tend to reach for a studio in software like Altiverb – which uses Impulse Responses of the actual environments. By setting up a stereo instance on an AUX track and assigning your room plug-in, you can use a bus to send some signal to it. What this does is create depth and distance in the mix. The amount of depth and distance is easilys controller by how much signal you send to the plug-in from your guitar track. I find with the edgier, harder sounding amps, drier is often better for in the face tone. But with a cleaner, blues type sound; you can typically get away with using more room ambience.
Tip 5 - find the right balance
Often, what I'll do is push the above room and echo techniques past the point where they sounds good, then pull back the bus send back a bit until your sound sits right in there. This will of course change depending on the type of song and what you're going for, but it works more often than not. To check it, just mute the echo or room AUX track and listen to the comparison of dry signal versus effected signal. Then you'll really know its right.