Tube amplifier will be HIGHLY sensitive to complex impedance of the speaker.
Only way for this to make any sense (to have some quantitative dimension) with tube amps is to either measure amplifier response with actual speaker as the load.
Or to take several representative speakers (or one reference one) and measure speaker complex impedance (with box, crossower and everything)
and than design/make a "simulator" complex impedance analog for that speaker (complex network built from resistors, inductors and capacitors) and use that for measurement.
Doing that is quite complex project itself, and will yield a rough approximation anyways.
Modern solid state bass amplifier will typically have a preamp that will do all of the sound shaping, and a power amplifier that will be wideband, pretty much like the HiFi one, but with less stringent requirements on distortions. Any excessive frequency response deviation from flat will be defect. It won't and shouldn't have any real impact on the sound.
If you want to measure preamp frequency characteristics, tone shaping and such, for that PC sound card and software is way better tool. Software will take care of everything, doing measurements automatically. There are many free and paid for ones to choose from. And voltage levels are not high, you can make small attenuators with normal metal film resistors if you don't want to overdrive sound card inputs.
With an 8 bit scope, you cannot measure less than something like 0,12-0.15% distortion, probably a bit worse. With sound card you can measure way better than that, even with the one built in on the motherboard. Easily 0,002 %, or better, depending on card.
So for just testing power amps to full power and such, there are cheap wirewound resistors (inductivity here is not a problem, quite the opposite) , and you buy few of them so you can combine them for 4,8 16 OHm and so you can test stereo at the same time... If amplifier is connected directly to the speaker, an air coil in series with resistor will be better "simulation" of speaker. If you want to be close, again, measure the speaker and you can than simulate it quite easily good enough.
Just to make it clear: speaker is freakishly complex load. It not only has a coil in there that is resistive and inductive, it is an electromechanical system (it is basically a linear drive DC electric motor/actuator) so not only the coil will have it's electric impedance in stationary state, but when actually moving, speaker will create counter EMF force, that will be dependent on resonant frequency and acoustic impedance matching to the surrounding air/box.. That will "mix" with electric impedance of the coil to make speaker impedance parameters.
It gets really complicated real quick.
So you have to figure out what exactly do you want to accomplish and than go from there.