Don't use a "D" amp or a bridged amp of any class. Best is to use a good A/B chip amp in Mono.
The LM3886 is perfect for this task, because is has no distortion, is linear, unconditional stable and short protected in any way. For a measureing amp you should not use it's voltage potential, but go for a lower voltage, like a 15-20 Volt dual supply. You do not need that much power for measuring. This will keep the heatsink small, the capacitors cheap and cost for a transformer low.
There are no such finished amps of this kind I know of, except for very pricy professional offers.
There are other amp chips that work as well, but the LM3886 seems the best IMO. Use a steel enclosure if possible, to keep noise down and have a stable base for in and output.
This is a basic, well working and cheap example I have used, you find it at Ali, Amaz. ebax etc.
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Because sooner or later you will be running into ground issues when using it with a sound card. Maybe not you, but a beginner. Why provoking smoking his gear?
Amplifiers using the LM3886 and various TDA7293 and similar chips are not ideal for these applications.
These types of amplifiers are significantly more sensitive to oscillation and other problems, and can even fail or burn out. The problems and experiences encountered in various DIY projects speak volumes, and often even minor component changes or slightly different capacitor values have fatal consequences—I can confirm this from my own experience.
As a measurement amplifier, this is an absolute no-go for me.
With the AliExpress boards, there's the added issue that most are shipped with counterfeit amplifier ICs, especially when the entire board is so cheap that it can't possibly be an original amplifier IC. There are countless threads about this as well.
The goal is to measure the frequency response of loudspeakers, which is usually done with microphones, so there's no grounding issue with the measurement amplifier.
Getting ready to do response measurements on my first speaker project in 40 years, and new to the measurement arena.
I need to find a small amplifier for the tests. Don't care about sound quality all that much as far as listening is concerned but needs to be pretty linear. Only need a few watts. The reviews here aren't encouraging. Is there anything that fits the bill that is both small and reasonably priced? I could home build one if I could find a design, but wouldn't want a large secondary project.
And bummer on the Parts Express story. I buy a fair amount from them (and Madisound).
The TPA3251/55-based Class D amplifiers are the complete opposite. These amplifiers are extremely robust and can withstand a lot. I've built and tested over 50 amplifiers of this type – from AliExpress boards, self-assembled boards, evaluation boards, and complete amplifiers – sometimes in extreme situations and with critical impedances, but not a single one has failed on me yet.
The Mini 300 recommended by
@JSmith should be the ideal amplifier for you, as it's unbeatable in terms of power, resolution, and measured performance for the price. Furthermore, its frequency response is exceptionally good, especially at low power levels.
The next step up would be the 3E Audio A7 (not A7se), which is even less sensitive to low impedances, but I don't think that's necessary unless you're building extremely impedance-sensitive speakers that drop well below 3 ohms at lower frequencies.
Regarding crossover design, I'd like to draw your attention to the possibility of using active systems, which makes much more sense these days than sinking your money into passive components.
Furthermore, it simplifies the design process considerably; you avoid all the negative effects of passive components and can control everything.
