Ronny, it is funny because I was juts about to carve my further observations about the Sound of my monitors. Last night I was listening Rigoletto by Callas and Gobbi on my monitor. I come to an interesting observation that I would like Monitors to sound good but I never thought to have them to sound “serious”. With all beauty that I am getting now from the haled SL600 I would do not compare their capacity to Makondo. Let be perfectly real about it. In order to make SL600 do not have that very pleasant but still alien tone at upper bass they should be crossed at ~150Hz. At that frequency my LF section (one 21W/8555 driver) does not work well and I end up with no real upper bass. Now I crossing SL600 at ~65Hz and the LF spread to 100Hz and still there is no upper bass in the room. I mean it is very good sound all together but if you flick the switch and let Macondo to kick in then it become kind of funny…
The thickness of the upper bass with SL600++ is there and the only known to me way to deal with it is to make a 2-chanal LF section – separate for upperbass and separate for the lower bass. I know that I will not be doing it and therefore I do not even try to get from SL600 more then it does. You see I was looking for monitors do not do bad things, not necessary to do the things very-well. Frankly speaking the SL600 have already surpass my expectations (after buying into 5 well operating but crappie sounding tweeters!)
The 4" Fostex in a sealed "box" with 25w/8565 should not be a wishful thinking of you cross the Fostex at ~ 700 cycles and add an upperbass channel, so you are on the very correct direction. I do not have a lot of experience making box speakers and I would not be able to advise you. You might try the Scan-Speak drivers, they always were very nice. The SS 18W-8535 was use in ProAs 2.5 and it did well. My only concern that the rubber suspended SS have very freakish transients at higher frequencies. I would probably go for more “ringy” drivers – some kind of miniature version of Altec 515… I really do not know what to suggest. Since you in Europe you might try to fish in German ebay a pair of 5”-8” original Görlich drivers. They have their cone that it not suppressed by the suspension but do not let them to go all the way down and kill them at 100Hz.
The Focal Utopia 165A was mystery. They did phenomenally in my car but they did very poor in my monitor. You might also try catching on Ebay an older version of JBL LE8. it was well sounding driver. You might work with it dumb aluminium cone and have difficulties to low-pass it but I would defiantly try it. The Danish company AudioTechnology does very good drivers. I think Bruno for Verity then them and few others. I would defiantly dive in the pound of their drivers if I do an boxed upperbass.
The Scan-Speak 15W-8530K might be very interesting chose but they are around $500. I always say that if they are good and useable then they might cost 10 times as much but if they are not good for your application then they are not worth even $10. Can you try them without paying $500?
I’m mentally searching the collection of my driver in my storage and I realise that I have no direct radiators for box solutions. I have large coalition of LF transducers and compression driver but in the lower MF I never looked at anything else besides the horns… My Fane 8M would feet your fill but it is not good as direct radiator. Although it has a very good tone as direct radiator but it severely compress sound with being loaded into a horn…What is boils down: the question: what commercial box speaker has good upperbass… Well, if I know an answer to this question then I would not do horns…
I think if I was doing “fundamental channel” ($100-$1000 for monitors then I would go for a sealed array of 4 X 6”-8” drivers trying to spread them as tall as possible. The larger radiation space would be the better they would load room. Still I would not go too high as it would crate time discrepancies at HF, I would say 60” high no more (however, it would create a huge problem, perhaps irresolvable with a “half ass project”, for the implementation of the enclosures…). I do not think that Large Sound might be emanated from a single driver, in addition the arrays have own advantages…. Well, now you understand why I am not wiling to go into this direction with my SL600 project. Perhaps is you find any “simple” solution then I would replicate it…
Sorry, I posted a long replay but I feel that I am completely no equipped to discuss this subject. The Tanglewood broadcast of Requiem is too boring and I entertain myself by replying to you during the performance…. How bad should this play be!?