Assuming all gear is functioning normally, the simple advice is "To avoid clipping the preamp, turn the pre-amp Gain knob down until the clipping sound goes away." If you have an active bass, you may need to set your pre-amp to handle the hotter input.
The Elf is 130 watts @ 8ohm and the Delta is 110 watts @ 8ohm. Not a lot. You also need to consider whether the preamp is clipping, the power section is clipping, the speakers are distorting, or some combination of the latter—all three or any two.
Here is what I do to find maximum volume without clipping or speaker distortion.
Get ready:
No pedals of any kind, fresh 9-volt in active bass, if applicable.
Set your pre-amp input to handle the active bass, if applicable.
Set all EQ to neutral, including any active EQ on your instrument.
Turn pre-amp Gain all the way off.
Turn instrument onboard volume on full.
Turn amplifier Volume all the way up.
When testing, use the (presumably) low pitches and manner of playing that cause aforesaid clipping and/or distortion.
Test:
1) Play and gradually increase the pre-amp Gain.
2) When you hear clipping or distortion, gradually cut low end EQ as you play until clipping/distortion stops.
3) Resume playing and carefully increase pre-amp Gain until clipping or distortion returns.
Repeat steps 2 and 3 until no clean volume increase is possible.
If the tone lacks low end, slightly decrease mids and highs and increase Gain, if possible. Cutting mids much will be counterproductive, as that is where the meat of your tone lies, and a sound without mids will soon disappear in a real world live band setting. You are lowering gain when you cut mids in most cases. Cutting EQ is actually cutting pre-amp Gain for a given frequency band—though this depends on the pre-amp. For example, I have an external preamp (pedal) which I use a lot. Its EQ is boost only. The more stages of Gain and active EQ you have, the more complex and tedious this can become!
Also, don't forget that the lowest pitches are not the ones you spend 100% of your time playing. Cutting midrange will reduce the lowest partials of pitches in the middle and upper-middle range of your instrument.
Beware that the Delta has a built in overdrive that kicks in when the Gain is at 100%. Also beware that the Gain control on the Elf does a some unusual things, too. FYI from the Elf manual:
"GAIN rotary control
This is to set the gain of the input stage of the amplifier. The setting of this control is the single most important on the whole unit. There are three regions of operation of the control, which can be visually confirmed by the activity of the signal LED.
SIGNAL LED
When the LED is green, the instrument signal is passing through the preamp without any compression. When the LED begins to flash red, more and more compression is occuring in the preamp as the GAIN control is advanced. The compressor also tracks changes made to the EQ; therefore, an increase in an EQ setting will require a decrease in the GAIN setting to maintain the same amount of compression."
