Joe X

Depends on what 18 and the maximum dimensions of the trunk. (h,w,d) 89

You can use a 1500W RMS amp no worries, just no clipping, subsonic set to 28Hz, careful with bass boost and other tone controls after setting gains.

So here is a suggestion, power: to sub(s) rated, amp subsonic filter to 28Hz, minimum port clearance 3 1/2", figure is just an assembly guide: NOTE: If building outside you need to build a cardboard dummy box to test fit before building the real box. NOTE: Box can still be used with better subs for a future upgrade.

OP post the model for the speakers and also the model of the amp. Maybe you will need two amps but let's see.

I would definitely look into seat lift kits to fit a box of proper size for those subs, make that enclosure with 3/4" birch wood, dual layer for the baffle, proper bracing inside, specs are 4 cubic feet net 64 square inches of port area tuned to 32Hz, I believe the sub cutouts need to be 7.34". Depending on how much space you find to work with I could fine tune that spec to increase output and low end extension as possible.

So here is a suggestion, power: to subs rated, amp subsonic filter to 28Hz, minimum port clearance 3 3/4", figure is just an assembly guide:

That box is designed for SA-8s as a result the X subs are going to find themselves in a too small box with low port area and tuned too high, surely above the 36Hz specified as driver displacement for X subs is larger, maybe it will get to 38Hz or more and could experience some port noise. Used to a large box tuned very low you will definitely notice a loss in bottom octave bass. Regarding to your question, you can always strengthen the box but if it is made of cheap thin 5/8" wood then it is not impossible. Lastly the pre cut sub holes cannot fit X subs, you will have to work on the holes. Overall that box seem expensive and not adequate for the subs. I would lift up the seats and do a proper custom enclosure for those subs and even then you probably have better bottom octave bass right now than you will ever get with some 8s.

Should be fairly easy to determine what's at fault just swap parts, you have another amp, test with that, use another sub and test, check your gains, do it fast so that you can send the product back for warranty if necessary.

RFP are HE2 not HX2, driver displacement are not the same.

Most important is to check the model of your alternator to make sure is actually 120A, if not what I am saying is not valid, secondly you should get an AGM battery of decent capacity, probably that should be enough to reduce the dimming, a big 3 upgrade is recommended, take a look at the link below to read some more on the topic: https://learn.sonicelectronix.com/5-steps-to-eliminate-headlight-dimming/ The other way to to help your electrical system is by using higher diameter subs which are much louder for the watt, a single 10 sub in a big vehicle will make you up the volume level a lot to get decent output, a 12 sub can gain you 2-3 dB just by merit of size, A 3dB difference means you can be as loud with a 12" on half the power that a 10" sub need.

Your vehicle is supposed to have a 120A alternator, if that's the case your electrical should be able to hold at 13.2V so as long as you only play with your car on you are good to go, to reduce the dimming substantially you could upgrade your battery and add a capacitor at your amp terminals.

13.2 V is nothing to worry about but if you are stressing a lot your electrical things may get worse over time so you may want to list what subs and amp models you have and what is the vehicle. That should be enough info to tell you if you are likely to have problems going forward or not.

Don't actually know what are you saying but if the lowest voltage you ever get is 13.2V in your car you might as well do nothing unless you plan to upgrade your audio.

You can play a 5 or so second 40Hz tone and up the volume level until the issue presents itself. Use a multimeter to measure (while the tone is playing) voltage between the negative terminal of the battery to the stud to see how good is your chassis ground for example. It should measure close to zero volts Then to the ground terminals of your amps and then to the positive terminals of your amps, you can do the same to any point of interest including the drop at the battery terminals themselves. If when the tone is playing the voltage drops significantly at your battery terminals then your electrical is not working right. If your wiring is right you should see most of the battery voltage drop at the amps terminals and very little at any other point, if that is the case your wiring is good. Hopefully you get the idea.

Many mainstream brands feature that "signal sensing" turn on you want, a notable brand would be JL Audio, many amps from them are capable of that. But that feature is not available in most high power amps (if not all) so usually you ad a low cost device between the amp and the head unit that will both convert to "RCA level" the speaker outputs of your head unit and create a "turn on" signal for you that you can use with one or more amplifiers that you may have, check out the following link: https://www.audiocontrol.com/knowledge-base/how-to-power-and-turn-on-the-lc2i/ If you happen to find the high power amp with that "auto turn on signal" please let us know which it is, so far it has eluded me.