What could be the differences between the Spendor A4 and A7 in terms of sound quality?

As a recent A7 owner and exhaustive researcher, I will throw in my 3 cents for both the OP and @glow_worm:

1. I have not heard the A4's but I am not aware of anyone who does not think that the A7 sounds better than the A4. The bass port, larger cabinet, and dampening are the main reasons. That’s an easy binary if cost is not a big issue.

2. D7(.2) is definitely a step up technically. But some don’t like the analytical top end. I am personally tempted to try the D7.2 but early reviews suggest it’s not much different. Even leaner if anything.

3. Tubes go very well with modern Spendors. I am using a 48wpc Line Magnetic LM-805ia and the synergy is amazing. Note that the smaller A4 is less sensitive and on the flip side, while the D7 is a bit more sensitive it has a lower impedance and thus a bit harder to drive.

So if you are concerned about the PL 34 watts the A7’s may be the safest match. That said, tubes might also mitigate the extra sparkle on the D7’s and bring it to perfection. I know there are others who happily run tubes with the D7’s.

FYI: The Spendor A line do have grilles but it is an option!!
https://www.spendoraudio.com/wp-content/uploads/Spendor-A-Line-Data-Sheet_June19.pdf

Personally I think you neither want nor need grilles for the A series. Though it is chintzy not to include them.

Room size may dictate your choice as well in terms of how much bass, and thus speaker, is appropriate.

the A7's, in my 13 x 23 room, produce superb bass at times and are pretty easy to position. I have the front baffle about 26" out, positioned along the long wall. I actually seek out music with lots of upright bass now. Not as chest thumping as some other speakers but they dig deep and very articulately. Almost like a larger sealed box.

I don't think the A4 can go there with the simpler bass port, D7's have even better bass of course but may need to be further out as per @kren0006 above, again depending upon room.

I think the various opinions on how detailed or forward or sparkly or bright or whatever term you want to use for Spendor are interesting.

Here’s my take on it.

For decades, Spendor has had the reputation of producing very laid back speakers. I have read all of the professional reviews on all Spendor speakers going back 10-15 years, and what you see over and over, at least in the background portion of the review, is a description of the brand as belonging to the "pipe and slippers" camp, or the "comfortable pair of shoes" camp, or similar descriptions. That seems to be the prevailing opinion of many of these folks who review speakers for a living.

Nothing wrong with that - it’s probably what helped the brand survive and thrive for 50 years now, and build the following they have.

But as that old reputation pertains to the current lineup, I think maybe it fits (as you’d expect by the name) moreso with the "Classic" line (which I haven’t heard as much of), than with the current A Series (I’ve heard all extensively), and even less so with the current D series (I own).

Some may say the D series really departs from the long-held reputation, and maybe that’s right.

Think of it as a spectrum from dull to overly hot, 0 to 100, if you will, with precisely neutral at 50, just for argument’s sake.

Maybe the historic reputation of Spendor put it in the 25-35 slot on that spectrum (just a guess, not claiming to be a super authority on this). Maybe the A series now is at 35-45 on that spectrum, and maybe the D series is now at 45-55.

My point is that (IMO) it is wrong to consider D series to be a "forward" or "bright" sounding speaker in the universe of all speakers. Brands like B&W, Paradigm, Focal, Klipsch and others based on my listening auditions (note, my opinion only -- I’m sure some may disagree, but that’s what I heard) rank above 70 on my hypothetical "hotness" scale (again, just my opinion), and Spendor D nowhere near that.

So are Spendor D more engaging and lively than historical Spendor? Sure, but keep it in perspective, Spendor D probably now right in the sweet spot of neutrality around that median 50 on the dial.

just my opinion. Nothing scientific about any of the views described above, just pontificating.

For those not familiar with the lines, the D uses a Spendor proprietary LPZ tweeter that differs from the tweeters offered on A and Classic lines. Spendor designed and manufactures the LPZ themselves, making them one of a fairly small number of speaker manufactures that design and manufacture without 3rd party help all drivers of the D line.