Ronnie wrote:

The next big problem with sound here I think is a difference in sound level between upper bass and MF/HF. Upper bass seems louder (and ought to be). Would you help this electronics-handicapped guy to pad down the upper bass horn with for example 6dB? Do I just put a resistor before the coil?

Ronnie,

There are no needs to be funny about it and there are no needs to get a daytime job.

A phase tester and RTA analyzer are absolutely obligatory thighs to employ and they are inexpensive and very simple to use. A simplest phase tester you might get for $50 and it will be absolutely enough and you will be using it to the rest of your life.

With RTA you would not need anything fancy. There are a number of computer based RTA and some of them are extremely good and very cheap. For instance you my try the John Murphy’s TrueRTA.

http://www.trueaudio.com/

It is very simple analyzer, cost juts $100 but it might be VERY powerful and indicative tool if used properly. The TrueRTA is 10Hz to 48KHzm, 1/24 octave.  You would need a microphone to go with it and nowadays you might get for sub $50 a very-very good instrumental microphone. Interesting that with tools like TrueRTA you might not need any good microphones or good sound card - you might calibrate them off.