"I have purchased a Peavey PC1600x controller which I am happily using as a LARC unit for my lexicon PCM81.  The only thing I cannot manage to send is a value for MIDI tempo. I can tell the Lexicon to receive the tempo via a CC message but still it isn't reading it."
The Lexicon PCM-81 Tempo Source (location 0.2 in the matrix) may be set via one of three methods: 
1. internal rate (location 0.0 in the matrix) 
2. front panel tap button 
3. incoming MIDI clock 
Note that MIDI CC is not a valid source with which to set the tempo of the PCM-81. (pp. 2-19 through 2-20 in the manual.) 
"The PCM81 is supposed to read the 0-127 MIDI range and apply it to its 40-400 bpm clock but somehow it isnŐt happening..."
Actually, the PCM-81 can use Tempo Rate as a source in its modulation matrix, and it maps the 40-400 tempo to a range of 0-127, but the reverse is not true. (p. 5-7 in the manual.) 
Also, if you go into your mod matrix and choose a MIDI CC as a source for one of your mod routings, notice that Tempo Rate is not an available destination for the routing. 
In short, there is no way to control the Lexicon PCM-81Ős tempo by means of a MIDI CC value. 
however... 
Since the PC-1600x can send MIDI SysEx, we can use it to control the PCM-81 tempo...it just gets a little more complicated. (btw, this should work for a PCM-80 also, but since I do not own on of those, I cannot confirm it.) 
First, we need to make sure the PCM-81 is set to be controlled via SysEx. Check to make sure the following are set properly in the PCM-81 matrix: 
Control Mode (1.2) System Tempo Mode = GLOBAL 
Control Mode (3.1) MIDI Receive = OMNI 
Control Mode (3.4) MIDI Automation = ON 
Control Mode (3.4) MIDI Target ID = ALL (127)** 
Control Mode (3.7) MIDI SysEx = RECEIVE ON 
Control Mode (3.7) MIDI Device ID = 0** 
** use the Load/* key to access these params when positioned at the address in the matrix...e.g., (3.4) will toggle between MIDI Automation and MIDI Target ID. 
Now move to the Peavey PC-1600x. In the setup you have built, select and edit the fader you were trying to use for tempo. Change its settings to these: 
Function: String 
String Sequence: F0 06 07 dv 0B 00 20 00 pr pr pr pr F7 
Min: 0 
Max: 360 
Param Format: 4Byte, Nibs, lo->hi 
Name: Tempo (or whatever you like) 
Here is how the string is defined: 
F0 = SysEx message ID 
06 = Lexicon ID 
07 = PCM-80 ID (81 will also recognize this) 
dv = Device ID 
0B 00 = SysEx Message Type = Parameter Dump 
20 00 = Global Tempo Value 
pr pr pr pr = lo to hi nibble-ized Decimal Tempo Value 
F7 = SysEx End-of-Message 
(note that in the hexadecimal string, the "0" glyph is a numeric zero, not the letter "O.") 
Save that and give it a go. Movement of the fader in question should change the tempo, and you will see the number inc/dec if you are in the Tempo Mode, (0.0) Rate location in the matrix. You should also see the tempo LED blink rate change (if you have it set to blink). 
Hope that helps! 
blessed be, 
aeon