How loud do you need to play? I find that a rig rated for at least 124dB is usually enough for my needs. IMHO, it's not always loud enough, but I am not willing to turn up much beyond 124dB.
Keep in mind that 124dB is my number...your number may be higher or lower.
Power Ratings:
The TH350 is rated 175W at 8 ohms and 350W at 4 ohms
The TH500 is rated 250W at 8 ohms and 500W at 4 ohms.
If you know how much SPL you need, you can figure out the minimum Sensitivity required to reach your goal.
Required Sensitivity = Target SPL - Decibel Change.
Let's say your Target SPL is 124dB (like mine).
To figure out the required minimum sensitivity, we need to know how much of a decibel change occurs between 1W and the usable power we have available. Usable power is the lesser value been the amp's power rating and the speaker's power rating. So if you have a 400W amp and a 300W speaker, the usable power is 300W. Or if you have 250W amp and a 400W speaker, the usable power is 250W.
Here are the decibel changes based on the TH350 and TH500 power ratings.
8 ohm cab with TH350 = 1-175W = ~22.4dB change
8 ohm cab with TH500 = 1-250W = ~24dB change
4 ohm cab with TH350 = 1-350W = ~25.4dB change
4 ohm cab with TH500 = 1-500W = ~27dB change
Note the figures above show a ~3dB change when you double the power...this is a good rule to remember.
175-350W = +3dB
250-500W = +3dB
Now if you know the cab's Sensitivity Rating measured at 1W/1m, you easily figure out how loud it will play, you simply add the decibel change from usable power to the Sensitivity rating.
For example in this post:
https://www.talkbass.com/threads/bu...tone-hammer-350-enough.1636793/#post-28118853 it is shown that the TH500 with a 4 ohm DB410 makes ~128dB with 500W. The decibel change from 1 to 500W is 27dB and we simply add this figure to the 101dB Sensitivity rating to get 128dB: 127 + 101 = 128dB
Back to our required Sensitivity Ratings based on our desire for 124dB minimum.
Here is the formula:
Required Sensitivity = Target SPL - Decibel Change
8 ohm cab with TH350 = 1-175W = ~22.4dB
Required Sensitivity = 124 - 22.4 = 101.6dB min sensitivity rating
8 ohm cab with TH500 = 1-250W = ~24dB
Required Sensitivity = 124 - 24 = 100dB min sensitivity rating
4 ohm cab with TH350 = 1-350W = ~25.4dB
Required Sensitivity = 124 - 25.4 = 98.6dB min sensitivity rating
4 ohm cab with TH500 = 1-500W = ~27dB
Required Sensitivity = 124 - 27 = 97dB min sensitivity rating
The Aguilar DB410 is rated at
101dB 1W/1m and it's available in 4 or 8 ohms. The power rating is 700W. The sensitivity rating is high enough to meet the 124dB goal in either impedance with the TH500. But you need the 4 ohm version to reach the goal with the TH350. The 8 ohm version makes 123.4dB calculated at 175W. Perhaps it's close enough?
The Aguilar SL210 is rated
97.3dB 1W/1m and is only available in 8 ohms. The power rating is 400W, but neither amp makes 400W a 8 ohms. The sensitivity rating is not high enough to meet the 124dB target at 8 ohms with either amp. Or you could say, neither amp make enough power at 8 ohms to produce 124dB with this cab. Even worse, the SL210 is not quite capable of 124dB without exceeding its 400W power rating. Calculated SPL at 400W is ~123.3dB.
If you find a 4 ohm cab that you like with at least a 97dB 1W/1m sensitivity rating, the TH500 barely meets the 124dB target, as long as the cab can handle 500W.
If you choose a cab with a sensitivity rating of 97dB or lower, you may need to consider running a pair for high volume situations.
When you add a matching cab, usually it's connected in parallel. The combined impedance is 1/2, power handling doubles, and sensitivity goes up by ~3dB.
So a pair of SL210 would be rated 100.3dB 1W/1m, the combined impedance is 4 ohms, and the combined power handling is 800W. Note the required sensitivity with the TH350 at 4 ohms is 98.6dB, so a pair of SL210s will exceed our 124dB target with this amp. Calculated SPL is ~125.7dB
With the TH500 and a pair of SL210s, calculated SPL is ~127.3dB.