Budget Strategies forTightening up Bass?

Based on the impedance curve of the Prelude http://www.stereophile.com/content/silverline-audio-technology-prelude-loudspeaker-measurements, Silverline may not be the best match for a tube amp.

Oh, really? Here's a direct quote from the manual:

About Power Amplifier

The standard impedance of all Silverline Audio speakers is 8 ohms. Their exceptionally stable impedance loading favors single ended tube amplifiers, as well as low powered solid state amplifiers.

Care to retract your statement?

My AV system is set up in a large irregularly shaped open living room. I found that walking around room with music playing helped me find the bass nulls and peaks. Changing my listening distance from speakers significantly affects bass response. Moving my speakers just a few inches closer together made a big improvement in my room. A bass trap in the corner of the smallest part of the room was also an improvement. A new stiffer floor help also helped as did my 100lb sand filled speaker stands I built. Jim Smith's Get Better Sound has a lot useful information on this subject.

I'm with Bigby on this one - TREAT YOUR ROOM! Build very thick DIY Fiberglass filled bass traps with a generous air space (e.g. 6"-8") behind them. I'm sure your $350 will go along way of buying some lumber and OC701 fiberglass or equivalent and some cloth to cover them.

Room treatments will put to shame (I'm exagerating on purpose to make a point) any kind of 'tweaks' associated with cables or amps or tubes etc in proving you with sonic dividends that far exceed your $350 investment. You will notive that the more elaborate systems here on A'gon usually have acoustically treated rooms. Coincidence? I doubt it!