Complex Dummy Speaker Load Impedance
For starters, if your amp is sensitive to the load then any load that is different from the "actual load" will not give you the results you want.  While an actual speaker (in the actual enclosure) will provide a complex load, any other speaker in any other enclosure will provide a different complex load.  In essence, the graph of impedance vs. frequency will be different for the two loads-- in some cases significantly different.  All of the notches and peaks in the graph will be at different places.

If your amp is sensitive to the load, then placing a significantly different load on it from the actual load will be no more valid than using a simple resistor load.  

You should ask yourself if an amp should be sensitive to the load.  Most people (including myself) would say that it should not be.  But there are some niche applications where some other attribute is more important than audio clarity-- like low power or intentional distortion.  

One important reason to not have an amp sensitive to the load is that the load will change over time.  Speakers age.  Components age.  Sometimes speakers get damaged and replaced.  If your amp is dependent on the speaker then it will not be guaranteed to work over time.

If your amp is sensitive to the load then the audio quality will also be load dependent.  That means that you must test with the actual speaker.  Additionally, the enclosure that you put the speaker in will change the loading-- so you must put the speaker in the final enclosure.

In my opinion, there is no point in making a dummy load for this amp.  The dummy load will not match the final load, so you don't know if your amp will be stable or even work properly with the final load.  You will also not know if it sounds good with the final load.  Use the actual final load, or just a simple resistor load.

If you choose to use the final load, but you don't want to be blasting out noise all of the time then put the speaker (plus enclosure) into a sound-isolating chamber.  The bigger the chamber the better, but it should be at least 5x the volume of the speaker enclosure.  A "cheap" way would be to buy a used refrigerator and line the inside with rigid fiberglass panels.  A used fridge can cost as little as US$150, or much less if it is broken.