miniDSP makes a variety of products, the 2x4HD mentioned is a device that can implement eq, delay, various filters (popular for setting up subs particularly). I'd also get a measurement mic (the Umik-1 mentioned is another miniDSP product) rather than an spl meter, or a Dayton UMM-6 is another (you can still take spl measurements but a lot more too). REW (Room EQ Wizard) is great for taking measurements but also can generate tones and can even implement some suggested eq from REW with something like a 2x4 unit.
Getting into more complicated testing is where you might employ an oscilloscope, never went that direction myself. Multimeters can be handy for a variety of uses, but there are different types and quality levels to consider depending on your goals.