Output Impedance and Speaker Impedance

Looking at the spec for the MA7900 I'd guess you're probably okay. I don't know the mechanism they use to achieve 200W into all different impedances.
The difference between the speakers' sensitivity is 4dB which means the less sensitive speakers will need 1.6x the voltage to achieve the same SPL.
For the amplifier to output 200W into 4Ω then the RMS voltage needs to be 28.3V drawing about 7A. For 8Ω the figures will be 40V drawing 5A current. So assuming the amplifier controls the output voltage to match the output levels then we might expect that the 8Ω output puts out 40/28.3 or 1.4 times the voltage of the 4Ω output. 1.4 x the voltage equates to around 3dB. So this would explain the better level matching.
But now when you run at full volume you're putting 40V into 4Ω which will draw a current of 10A. In a lot of cases that would be a problem and you could end up with overheating but in this case the amplifier is capable of running with a 2Ω load so the chances are you are okay. If you put a 2Ω load into the 8Ω output you'd draw 20A which would likely put the amplifier into protection mode but is not worth the risk of trying.
Another way of achieving the same (or better) result would be to put a simple voltage divider (two resistors) across the inputs of the higher sensitivity speaker to reduce it's sensitivity by 4dB. You may be able to buy a 4dB attenuator to do the job for you.

Hope that makes sense.

the cone excursion is not halted, but over travels slightly on the out and the inward movement, in other-words it’s not as tightly control as Aerial would like it to be.

This is only partially true. A loudspeaker cone can be overdamped; looking at the impedance curve (https://www.stereophile.com/content/aerial-acoustics-5t-loudspeaker-measurements) we see that the impedance never dips to 4 ohms and spends most of the bandwidth at much greater impedances. This is why the 8 ohm tap sounds better.

Normally running an 4 ohm load on the 8 ohm tap will cause the power tubes to make more distortion and run hotter (ultimately failing sooner) since the load impedance is transformed by the transformer to a load impedance that would be half of that for which the circuit is designed.


But in this case this isn't happening so no worries!

Before you read the rest ubbcbus , the Aerial Acoustics 5T is not an overdamped design, it’s normal.

A loudspeaker can’t be overdamped electrically, this is why some of the best amps for bass are from class-D, that have output impedances in the milli-ohms, damping factors of 10,000 or more and why they are great for subwoofers.

However you can have too much damping if the electrical damping is combined with too much mechanical damping in "over damped" enclosure design.

A typical example was the Linn Isobaric speaker which had very lean almost no bass with big Krells of the era, who would have thought a Krell with no bass!!!!
I actually heard a Naim amp with very mediocre damping factor sound better than the Krell into the bass of the Linn Isobarics.

BTW: this overdamped speaker with underdamped amp, was a great marketing strategy thought up by Linn/Naim at the time, you had to get both their amps and speakers, one did sound right without the other in A/B’s at shops.

Cheers George

Thank you ALL for your responses. I'm relatively new to AG and lurk daily, but do not have much to add to the knowledge base so I don't post frequently. Really appreciate the knowledgeable veterans replying to my inquiry. Regarding the responses:

@millercarbon - not a home theater, only for music. I started out with the 7900 and the Aerials, but was pining for more bass (48Hz wasn't cutting it). Listened to a few sub-woofers but was not impressed with what I heard. I listened to the Triton's and was impressed with the bass response (especially considering the price). I know these get poo-pooed on the forum and that's OK, I know what I like. After reading up on AG about tube sound and sound staging, I decided to invest in the 275 knowing that it could be piggybacked on the 7900 to power the Tritons. Granted, it's an odd approach but it's synergistic in a sense. SS + tubes, good midrange and highs (especially voice) from the Aerials and good lower frequencies and sound stage from the Tritons... I've never been too conventional.


George- overdamped room for sure. Been considering taking up the carpeting and putting down cork floors.


 @bdp24 - M.R. amps may have different wattages for different impedance ? The 7900 & the 275 claim same wattage for all impedances as @pragmasi pointed out. I'm going to check in with Aerial to see what they have to say. 


@ pragmasi - regarding the resistors, the Aerials have 4 connection posts. +'s and -'s upper and lower. The upper and lower posts have  bridge connections  i.e. + to + / - to -. Right below the upper posts is labeled "midrange and treble" . Could these potentially be resistors or a bridge that functions in the manner you speak of ?   


@onhwy61 - Yes, both sets of speakers at the same time. I have a decent sized listening room 18x30. Its has 45 degree pitched walls, two dormers and a funky diamonded shaped cut out for the pitch of the adjacent, perpendicular roof. With 4 speakers, it's an acoustic circus but pretty enveloping sound. At medium to higher volume, if you close your eyes you cannot tell where the speakers are placed. 


@atmasphere  - do you think there is a chance to damage the drivers by overdamping them ? They're paper (papyrus if you're so inclined :) The bass when connected to the 4 ohm posts is less pronounced....  maybe another question for Aerial

Thanks again to ALL of you,

Uncle BB 

Thanks again George. As I get more in to this hobby, seems I'll need to do some reading on classes of amplifiers and speaker pairing. There's a lot to learn. I have a Unit Atom and when the firmware went awry with my 7900, I hooked up the Aerials to the Atom (40 watt amp) so I could listen to music. It sounded a lot better than expected. And conversely, when I hooked them up to the 275 they sounded very thin and anemic. Your point is well taken. 

Best regards,

Uncle BB