There isn't any magic; the out knob on the c-lator is the master volume for your amp from a technical point of view.
Together with the smoothing this unit does, you can have, at moderate sound levels, a well balanced tone.

The C-lator adds a slight compression (In a good way) and the Klein doesn't change the character of the amp. When using the these effects loop buffers the Level control becomes your overall master, IMO it handles turning the amps master up better than the klein

 At low levels with the C-lators level control (or overall master) at about 9:00 or so I could turn my amps master up to about half way (or 12:00).By balancing both masters the right way the amp got very responsive.

  At low levels with the Klein's level control (or overall master) I couldn't turn my amps master past 9:00 and get a the same results ie; the c-lator could handle higher master levels and the Klein didn't perform as well @ low levels

  IMO they're both good devices to add time based effects to your d-style amp. The C-lator will change the character of your amp and may be more pleasing sonically to use. Yet I still see the Klein as being a great alternative for the money ( At about half the price of most conventional stand alone tube buffered effects loops).

 Also a good mod for both devices is a series/parallel switch as you can get tons of versatility being able to use different effects that may perform better under parallel conditions. It gives you the ability to blend (or mix) the effect with the dry sound.

Just time based effects (Delay,Reverb,Flanger,etc) Just about anything that you'll want after the Overdrive. When you put time based effects before overdrive it sounds bad IMO.

The fx-loop of the amp is passive and has a high impedance; High impedance doesn't work very well with long cables. You lose a lot of high frequency.

The fx-loop of the klein/clator is low impendance and can handle large cables a lot better.
But don't go too long, you always will lose a bit of high frequency.

I agree with Rane008.
I have both and the C_Lator adds a nice warmth to the amp that the Klein cannot match.
IMHO one or the other is a must with these amps if you run time based effects.
A Univibe is one time based effect that sounds better not being in the loop, to my ears anyway

I just ordered a 50W Overtone Special.  This is my first experience with these amps.  I'm coming from a rig where I put everything in front of a Super Reverb.   With the OTS, I'll use the onboard drive, wah, delay, a reverb unit of some kind, and a Boss TU-2 Tuner.    Sounds like I'll need an effects buffer so I'm looking for suggestions: 

1.  Should I get the C-lator? or Kleinulator?  At present I dont have any rack gear, everything's on the floor.
2.  I vision my rig to be:   Guitar > Boss TU-2 tuner > amp input, then effects send > C-lator or Kleinulator > Budda Wah > TC Electronic Nova Delay pedal > Reverb Pedal> effects return.   Does this sound right? 

Cables:  10' cable from guitar to Boss Tuner on floor, 15' cable from boss tuner to amp input, short cable from amp to C-lator, 15' cable from effect send to wah/delay/reveb also on floor, and 15' cable from reverb back to effect return.   This is a lot of cables.. too many?

Appreciate the help, thanks!

While I appreciate Marin's videos on the C-lator, they really don't showcase the difference between it and the Klein. 

Now, I just got my C-lator back from the shop (shorted out because I busted a switch).  While it was gone, I was using the Klein.  Getting the C-lator back, I realized that it is capable of producing much more volume than the Klein.  It's has more depth.  I know that this is all subjective, but I thought I'd put it out there.