Assessing custom speaker performance

Last year, with lots of help from this forum, I put together a pair of three way PA cabinets (15"/10"/3"compression).

I was very pleased with the result, but I feel like some refinements ahould at least be attempted.

I have been running three-way speakers with active crossovers for many years, but this is my first venture into custom speakers. In the past I've always been handed cabinets with specific ratings and settings for EQ and crossover points.

I'm hoping for some advice on two aspects of this process. The first is simply this: what do you listen for when you've put together a custom cabinet and are looking for ways to refine the result? For example, do you listen to the drivers separately, or together? Obviously I guess together but also separately? Are there certain telltale signs that your phase plug is doing a good job or a poor job in this application?

Particularly with regards to the cubic volume of the housing for the mid range driver, which is loaded into a molded horn, how do I assess when the volume of the drivers enclosure is at its sweet spot?

The second advice I'm looking for a relates to tools. All I have at the moment is a modest Klein multimeter.

I could probably either afford a pro audio resistance meter or an RTA microphone. Or is there something else?

I have a limited budget, so please prioritize your answers to this, and if you recommend specific products, don't recommend the best product available, recommend the best value.