Falling interest rates typically cause the prices of dividend-paying stocks to rise, which then pushes down their yields. However, while the recent decline in interest rates has helped fuel big gains in many high-yield stocks, some continue to trade at rock-bottom valuations. Therefore, income-seeking investors can still score a high yield for a low price.

Three dirt-cheap yield-focused stocks worth considering are midstream companies Energy Transfer (NYSE:ET), MPLX (NYSE:MPLX), and Plains All American Pipeline (NYSE:PAA). Here's why income investors will want to give these energy companies a closer look.

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A bottom-of-the-barrel valuation

Pipeline giant Energy Transfer is on track to generate between $10.8 billion to $11 billion of adjusted EBITDA this year, about 15% higher than 2018's level. With the MLP's enterprise value currently at $92.1 billion, it implies that Energy Transfer trades at an EV/EBITDA multiple of roughly 8.5 at the midpoint of its 2019 forecast. That's at the bottom of its peer group, where rivals trade between 9.6 times and 12.8 times their EV/EBITDA.

That rock-bottom price doesn't make any sense. For one thing, Energy Transfer's financial profile has improved significantly in recent years. The company, for example, generated enough cash during the second quarter to cover its 9.3%-yielding dividend by 2 times. For comparison's sake, most rivals are comfortable with coverage levels above 1.2. Meanwhile, Energy Transfer has an improving balance sheet with a debt-to-EBITDA ratio trending toward its 4.0-4.5 target range. Add it its above-average growth prospects, and the company should sell for a much higher valuation. Since it doesn't, income investors can buy this high-yielding energy stock for a ridiculously cheap price.

Same story, different name

MPLX also trades at a low valuation given its current EV/EBITDA ratio of 9.6. That makes it the second cheapest pipeline company in its peer group. Accordingly, it pays an above-average dividend that currently yields 9.4%.

Like Energy Transfer, that low relative valuation doesn't make much sense. That's because MPLX also has a well-supported dividend given its healthy 1.36 times coverage ratio during the second quarter. Meanwhile, the company has a solid balance sheet backed by a low 3.9 leverage ratio.

In addition to its conservative financial profile, MPLX has compelling growth prospects. The company recently signed agreements to move forward with the Whistler project, which is a natural gas pipeline in the Permian Basin. It also recently agreed to become a partner on the Wink-to-Webster project, which is an oil pipeline out of the Permian. On top of that, it's expanding its gathering and processing operations in the Permian and its export capabilities on the Gulf Coast. Those expansion projects should enable MPLX to continue growing its cash flow and dividend at health rates in the coming years. Add it up, and MPLX's combination of a lucrative income stream and visible growth make it a screaming bargain these days.

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Tell me you haven't heard this all before

Plains All American Pipeline currently ranks as the third cheapest among the largest midstream companies at 10 times its EV/EBITDA. It's even cheaper on a free cash flow basis. With the oil pipeline on track to produce $2.80 per unit of distributable cash flow this year, and its units currently selling for $21 apiece, Plains All American trades at 7.5 times its cash flow. That's one of the lowest levels in the sector.

Again, that low valuation makes no sense. Plains All American, for example, is generating enough cash to cover its 6.9%-yielding distribution by more than two times. Meanwhile, its leverage ratio was 2.8 at the end of the second quarter, which is comfortably below its 3.0-3.5 target range.

Plains All American compliments its strong financial profile with enticing growth prospects. The company is currently on track to grow its cash flow per unit by 14% this year. Meanwhile, it recently added five new projects to its backlog, including leading the development of Wink-to-Webster. Because of that, Plains All American believes that it can grow its high-yielding payout by at least a 5% annual rate over the next several years. That's one of the many reasons why it's such an attractive investment these days.

Great income streams for an excellent price

Energy Transfer, Plains All American, and MPLX are the three cheapest large-cap midstream companies. It's hard to justify those low valuations, since these companies have healthy financial profiles and enticing growth prospects. Add in their high yields, and they're excellent options for income investors to consider buying.