What's a Leveling Amp ????

The original LA-2A is a tube based, optical compressor, meaning it uses a photo-electric detector cell to control the gain reduction.  This is the key to its sound and behavior, and is the reason why the controls are so simple.  The attack and release (and partially the ratio) are dependent on the program material passing through it.  There is a really great overview of how the LA-2A works here:
http://www.uaudio.com/webzine/2003/june/text/content4.htmlIt definitely imparts a sound on the material passing through, but I find that you have to be careful with them (at least the real analog versions) because you can hear it working if it's doing too much gain reduction.... which may be good or bad.  Otherwise, when you have it set up right so that you don't hear it pulling the gain down, it is a very smooth type of compression.

Usually when I'm recording bass or vocals, I'll go from the pre-amp through my Cranesong Trakker, with the Trakker set to one of the Optical settings, to get that kind of LA-2A thing going... but, only just a little bit while tracking.  When I'm mixing, I'll usually use the LA-2A or 1176 or Fairchild compressor emulations in the UAD-1, depending on what type of sound and vibe I'm going for.

For vocals in a dense mix, I'll usually use one of the HP filters from the UAD-1 Cambridge to get rid of any low end junk in the voice (or rumble from the room, if that's a problem) anywhere from about 80Hz to 100Hz on down.  Then I'll go through whatever compressor I'm going to use for that track, and then I'll usually add a nicer EQ after the compression to add some high-end back end, maybe some upper "air", and maybe cut a little lower mids if needed to help open things up a bit more.  My favorite EQ plug-ins now are the UAD-1 Neve models (1073 or 1081), and the UAD-1 modeled Pultec EQ is awesome for adding that air EQ (set the high frequency to 12Khz or 16Khz and boost that as needed).  Depending on the singer and microphone and such, adding that extra high-end may also bring out too much sibilance... if that's the case, I'll use the Waves De-Esser plug-in to pull back some of that.  Finally, if required, I may put a peak limiter on the end of the vocal chain just to give me an extra dB or two of gain and to keep that channel from clipping out.  The UAD-1 precision limiter is very nice, but sometimes I'll even use the old Waves L1 Ultramaximizer for more aggressive stuff.

The signal chain for bass is really dependent on the song and how the bass and kick drum are going to fit together, and how dense the overall mix is.  I'll almost always use some compression on the bass when mixing, either with the LA-2A, Fairchild, or 1176 plug-ins, depending on the sound I'm going for.  If the kick drum is going to have the low-end punch (around 50 or 60 Hz), I may roll off some of that from the bass guitar, especially in a dense mix where excess low end of the bass is just going to make it sound muddy.  I may bring out some upper mid-range, somewhere in the 600-1000 Hz range, if needed to help bring out a bit more definition in the bass, and maybe even a little boost around 2Khz to 3Khz to bring out some string noise.  Sometimes I'll even put the bass through some mild distortion to help generate some extra harmonics and make it cut through a bit more (usually only in a dense mix with heavy guitars).  Once in a while I'll also use the RenBass or MaxBass type plug-ins to generate some of those harmonics to make it seem like there is more bass without actually adding more low end.  If I'm going to roll off some low-end from the bass, I'll do that with a Low-Shelf EQ or High-Pass filter first, then go through the compressor, and then any EQ for tailoring the sound will come after the compressor.  Most of the time I record the bass direct, and then sometimes I'll through it through an amp model plug-in to give it a little more life in a dense mix... I like izotope trash for that, but am playing around with some other amp model plug-ins as well.

Steve