So I have my Mesa 400+ with my Bergie 4 Ohm 610. I plug that into the 2 Ohm section. Now I have my new 115 cab (which I am going to get soon ) which is most likely 8 Ohms. Now I plug that into the 4 Ohm jack, and all is good. But what I want to know is how much power is going to each cab?

According to the Mesa site it puts out 500 watts at 2 ohms. Also, here's an excerpt from the owners manual: Two 8-ohm, two 4-ohm and 2-ohm speaker jacks are provided on the Bass 400 Plus. The amp will produce its full power at all of these impedances, provided that the total impedance of your speaker cabinets is matched to the output(s) used. (Slight mismatches are not a major problem, but they will cause slightly lower output and may shorten tube life.) When using more than one speaker cabinet, remember that the total load impedance goes down as more cabinets are connected. For example, if you use one 8-ohm speaker cabinet, use one of the 8-ohm jacks. But if you use two 8-ohm cabinets, they will be operating in parallel and the total load will be 4 ohms. Therefore, plug each cabinet into a 4-ohm jack. If you want to use two 2x15 cabinets where each cabinet is rated at 4 ohms (which is how Boogie 2X15s are wired), connect both cabinets to the 2-ohm Jacks provided.

I dont get what your saying there . . . Anyways, the Mesa400+ is an all tube amp so always puts out its 300 watts, mesa states 500 watts peak, nobody knows where this came from, each of the power tubes it uses puts out about 25 watts (on a good day) For the soeaker impedances, if your just using the 4 ohm cab, plug it into the 4 ohm section (duh), if your just using the 8 ohm cab plug it into the 8 ohm section If you are using BOTH cabs, then plug them into the 2 Ohm section (someone want to confirm what im talking about? if it isnt pish, this is where i prefer the setup on SVT's, where you have a selector switch for the impedance tap)

It would be better if you got a 4 ohm 1x15, less confusion And as tash said, about 150 watts to each cab

I know for a fact that the impedances match perfectly if the 4 is in the 2 while the 8 is in the 4, it does not make a 2.6 load on the tubes or anything. What I wasn't sure about is how many watts would go to each cab, since on the SS gear I've wired an 8 and 4 together on the 4 gets 2x as much power as the 8. I was just not sure if it pertained to my amp, since the impedances would be balenced out...

As far as I can remember, it's cool to run the 400+ like this with a 4 and an 8 Ohm cab. Each cab will dissipate the same amount of power, because the 8 ohm cab will see a higher voltage than the 4 ohm one by a factor of 1.41:1 because of how the transformer works). Power is the square of voltage over impedance. The square of 1.41 is 2 so you can see that the voltage squared part of the equation for the 8 Ohm cabinet will be twice that of the 4 Ohm one and when you divide it out the power is equal. The amp will still only produce 300W of output power, no way around that.