If you are looking for minimal coloration or nearly transparent compressor with a fairly wide range of control and great metering, the MXR M87 might be just what you are looking for.
It isn't a compressor to make your tone "fat" or to bring on the "tone magic" associated with the likes of other compressors such as the Doc Lloyd Photon, Diamond, Pulp 'N Peel, Forest Green, Bogner Lyndhurst, or Cali76 CB. What the M87 does well is general compression with no real loss of highs or lows. It isn't "snappy" like the Cali76 CB, but it isn't dark either. Some might say more "sterile" but I'd say more natural.
Let's start with the LED's.
Second to the Empress I think the MXR M87 is a winner with how well the LED's communicate how much your signal is being compressed.
There are green, orange and finally red LED's depending on how heavy the compression is squashing the signal. Very nice. The LED's are very easy to read and are especially helpful if you are learning to dial in compression and understand how the controls are interacting. I suspect many are drawn to the MXR for this very reason and justifiably so.
The other controls are Release, Attack, Input, Output and a 4-way toggle dial for Ratio.
Starting with the Release knob, there is a fair amount of range with noticeable difference across the dial. Rotating the dial clockwise decreases the amount of time it takes for the compressor to release the compressed signal to its original uncompressed state. I really liked the Release knob set at about noon.
Turning the Attack knob clockwise speeds up the time it takes for the compressor to latch on. As others have commented and other reviews have stated, the Attack really does have a very narrow range. It is basically different versions of fast attack. I found I pretty much left the knob all the way counterclockwise to slow the attack as much as possible. I suspect most users will be totally fine with the range, but those looking for as much transient initial attack to come through will be let down.
The Ratio dial is a 4-position selector from 4:1 - 20:1. At 20:1 it does serve as a pretty good limiter. I enjoyed slapping the bass with the ratio at 20:1. 4:1 is great for overall tone sweetening and evening things out. To my ear, I didn't hear a whole lot of noticeable difference between 4:1 and 8:1 through my Beyerdynamic DT770's or via a Genzler rig.
The Input knob essentially controls the threshold at which the incoming signal is compressed. The more clockwise the dial is turned the faster the compressor kicks in. With all of my basses I found I needed the input knob set at 1:00 or more clockwise. Even my G&L L2500 with hot output. That knob is very sensitive though, especially around 1:00 - 2:00. I tiny turn goes a long way. If you want to make sure you've got it dialed in exactly the way you left it after transporting it around, you might want to mark your sweet spot with a Sharpie dot. It really is that sensitive.
I had no issue with headroom with any of my basses from a hot output G&L L2500 to my fretless Pedulla to a Sire P7.
The Output knob is used to make up any lost gain post dialing in your compressor or to boost above unity. There is plenty of gain on tap.
Inside the enclosure is a 9volt batter connector. There is also a little switch on the circuit board that turns off the LED's. So if you discover the LED"s provide too much of a light show, you can turn them off. I doubt you will.
The M87 seems well built overall. Not up to spec of some of the more boutique pedals, but those pedals also cost a lot more. My preference would be for larger knobs but what comes standard work fine. The Ratio switch knob does pull off quite easily so be careful with that one.
The stomp switch is not the silent click type and is true bypass. Power runs on standard 9-volt (no power supply provided). Side mounted jacks and power input. It is also quite small; the same size as all MXR pedals.
Power LED lights blue.
I was quite impressed with how quiet the pedal is. Maybe not as silent as the Diamond or FEA compressors for example, but certainly no issue with noise to really complain about.
Overall the MXR M87 is a good clear, quite transparent compressor with a pretty good amount of control. Certainly more than most in it's price range.
The LED's are icing on the cake. It's a good value and has a lot going for it. You really get a lot of compressor for the price. It sounds great, and has more control than a lot of it's competition. It's a versatile compressor. It's one of those compressors that you might not notice... until you turn it off. Some might say that's the best kind.
Retail price: $189.99