Whether it is an advantage or not to port the rear chamber depends on what one want to achieve.

If the drivers parameters is suited for the use in a ported horn then the result will typically be that the system can go lower when the rear chamber is big and ported but the efficiency up higher will be a little less than the horn with a small closed rear chamber. The response will also typically be a little more uneven. (How much depends on how well the driver is suited and how well the system i tuned.)

An example could be my 2 x 12 exponential horns.

They have a big rear chamber with ports in the front (next to the hf horn). But they also have a kind of a hatch in the back, making i possible to change to a small closed rear chamber. When used in port mode the system is tuned to 70Hz, making it useful as a standalone system in a small indoor environment. But when used outside in 2 or 4 Pi environment, the response gets quite peaky. Then the system can be changed to closed rear chamber giving a much flatter response and about 2db more sensitivity. In this case the horn is only useful to around 100Hz. I usually cross them at 112Hz when used with a closed rear chamber.

I could scan it in for you one day if you like, but that should be in another tread

I changed a few thing on the final design, but here is original drawing:

And then a slightly modified drawing with some measures:

 

They are made with 20mm panels. But if i had to build them again i would use 18mm ply.

The 12" speakers is mounted on a separate detachable baffle with round holes. Originally i would have made them with phaseplugs, but i tried one baffle with phaseplugs and one without and found that the drivers i used didn't go above 1000 Hz that well either way. Therefore i made the the baffles without phaseplugs and used a big 2" driver for the highs. Another reason for using a crossover frequency as low as possible is to keep the dispersion at a acceptable width. These long exponential horns has a very narrow dispersion if crossed over at high frequencies.

I know this drawing isn't enough to build an exact copy of the horns i build, but i wouldn't recommend doing that any way. You should tweak the design to suit your needs. Considering how low they need to go, what dispersion you need, what size you can live with and what drivers you will use.

At the moment they are loaded with Kenford PA 12" DYD300B-4-67. It was the driver the horns where design to initially. I have also got some B&C 12PE32, but i have choose to let the kenfords stay because they actually works unbelievably well. One should never trust the specs given for these cheep drivers. I think they are actually very similar to Eminence kappa 12A.

Actually i haven't tried to tune the ports high because my sims showed that it would give me a big peak in group delay around the all important kick frequencies. I have tuned mine to 70 hz and then i only use them in port mode when i can find some nice corners to place them in.  (this removes most of the dip)

About drivers for this type of ported mid horns. I would recommend not using hardcore horn drivers with very high Bl, because then the dip between the port and horn tuning will get to big. Again, something like Eminence kappa 12A would work well in these designs.

Do you mean if you put the port output directly in the horn or like I have done?

Problem is I have never heard a usb. But if you mean standard reflexes(no horn) then 3 times as many as a well design horn 

If you have usb's then why would you want to extent the response of the 12" horn down low?

About the kenford. I would not recommend it without further testing. I don't trust any of the specs given for it

But I would certainly recommend that one bought a single one and tested it and saw how much abuse it could take. And how well it performed in the intended design. Problem is that I have never had mine out with a bottom end that have had any chance of keeping up with them, so I don't know how they perform under high power conditions.

One thing I can say for sure, which could be an important issue in PA, they will not work forever in high humid conditions, because the cone is not coated.

"i always thought if you put your horn parameters in and add ap and lpt (for example 150cm² and 15cm length) it means the port goes outside the horn along the horn to the hornmouth, so the same as your tops in the pictures."

One thing that is also very important is the lenght of the rear chamber. (Quite long in my case, and that gives rise to some cancellation. Which is less dominant when used in a corner.)

- That was because I was not sure what you where asking. But I think that I understand now.

Making your mt121 ported will not be able to replace your usb's. My horns is a double 12" and longer than the mt121. And they can't do kick in reflex mode anywhere near as good as proper kickbins.

The only time I use mine in port mode is for small things indoor, where they are used as standalone for non bass heavy music. Or with a single dedicated sub for other music styles.