Adding an 8ohm tweeter box to a 4 ohm 215

[rockbobmel]

So I am pretty sold on keeping my Berg NV215 but I want to add some highs.

I ran it before with a biamp GK800RB, which has a separate tweeter amp.

Experimenting, for 10 minutes, I ran another 215 (4ohm) cab with my Sun 2000S off the optional 4 ohm tap instead of the 16 (The brown wire that comes from the OT), then I plugged a medium size Klipsch center channel speaker (2x4"+horn) (8ohms) out of the 215's cab parallel jacks.

The sound was very balanced between the 2 cabs. Since the Klipsch rolls off high, there was no distortion. Just more presence.

A lot of 4 ohm cabs have tweeter/mid/ etc speakers included in them (Including the Berg has a 6"), so that makes me wonder if it really changes the total impedance of the total load.

Is it safe to run this cab on a tube amp this way, namely in my Mesa 400+?

[Psycho Bass Guy]

This depends VERY much on the crossover and the drivers in  the "high box." When you run the cabinet in parallel, it doesn't matter how it sounds, the power is being shared evenly between the two cabinets UNTIL the crossover/physical limits of the Klipsch drivers come into play. It's not going to hurt anything to try it and just run off your normal  4 ohm output tap. If you start to notice any part of the amp running noticeably hotter or it being more distorted, switch to the Boogie's 2 ohm output jack.

[rockbobmel]

Awesome!  I found out a little more. 
It is a Klipsch Synergy SC-1, 2-way
Max (RMS) Output Power 100 Watt
-3dB Frequency Response 60 Hz
Nominal Impedance 8 Ohm
Sensitivity 95 dB
Crossover Frequency 2200Hz
Magnet Type neodymium

[Psycho Bass Guy]

The info you need to mathematically model what will happen is the power handling of the capacitors in the crossover and the frequency/impedance response plot of the drivers in both cabs, but it's much easier to just try it. Since the Klipsch is a 100 watt cab, you ought to be OK in terms of not overpowering and blowing it.  The 400+ is only 225 watts anyway and you're sharing the output with a big power eating 2x15. I doubt the Mesa's output transformer/power tube section (what you normally would need to be worried about with mismatches) will be strongly affected at all.

[rockbobmel]

Well it was a success last night.  I kept checking for excessive heat but it remained constant and fairly warm but I think running the berg made the amp work harder than the Bagend stack. The Berg was a proto (rat fur) model so it has stock Delta 15A speakers in it. The klipsch center just sat in there nicely without complaining.
Thank You Much! PSG